How do you correctly soften butter for spreading on bread? Is it simply a matter of leaving it at room temperature for a few hours, or do tools like butter conditioners do a better job?
If you want to turn your cold butter spreadable quickly, patience is about your only option. You can try microwaving it very briefly, and maybe on the "defrost" setting, but you run a strong risk of melting it too much.
If I'm thinking of the same thing you are, a butter conditioner is really just a unit that allows you to keep your butter in the fridge but still keep it closer to room temp--so that's more about storage than turning hard butter into spreadable butter on the spot.
A storage option that allows you to keep your butter fresh for a pretty long period, but still leave it out on the counter so it's soft, is a butter bell. You can look them up online--many shops have them. It's basically an inverted cup that sits inside a larger one with a little water at the bottom. The butter is packed into the inverted cup and stays put when you place the cup so the edges are under the water in the other cup. The water keeps air out so the butter stays fresh even at room temperature. You could do the same thing by mashing a bunch of butter into a teacup, then setting it upside down in a saucer full of water. If it's too hot in your house, though, the butter will slip out of the inverted cup, so I mostly only use our butter bell in the winter.
One thing to note about butter itself: Some butters are more spreadable at cooler temperatures. The Irish and French butters we can get here are much closer to spreadable at refrigerator temperatures than the regular US supermarket brands. I'm not sure what the difference is, though.
I agree with Bikeboy's answer, my mom uses a butter bell and loves it. I've come up with a way to soften butter though that seems to work out pretty well and is quick. I buy the short little stubby stucks of butter, put them on end in the Microwave and zap them for 5 seconds at a time. Each rotation in the microwave, I flip it over. It usually only takes 10 seconds, but sometimes I flip and go a few extra seconds. Oh yeah, and I do this IN the wrapper, you can feel the butter getting soft.
This process helps avoid the hot spots and gives a more even softening. Though the ends do stay a little bit harder than the middle, at least it doesn't seem to melt at all. Just don't hit the minute button and forget to flip it or you'll have a mess.
If you only need enough butter for a person or two, and don't need to soften the whole block (or keep the butter measurable by the little markings on the side of the wrapper):
Scrape your butter knife across the full length of the stick.
The butter will warm up from the outside it. As it sits at room temperature a thin layer of softened butter will form around the stick. By scraping the length of the stick you don't have to go as deep to get the same volume of butter, as you're working with more area.
Chopping it up will help, but will still need some time for preparation and waiting.
What I do is to use a fine-toothed serving knife for spreading the butter. Instead of cutting off a piece, I use the teeth to rasp butter off the surface of the butter stick. I end up with a very thin bunched-up butter sheet on the knife, which can be spread easily at uniform thickness on the toast.
Also, I find butter to fare best at 15-ish Celsius. It doesn't go rancid and too-soft the way it does if always kept at room temperature, and it is easier to spread than 4 Celsius butter. This is accidentally also the best temperature for many other foods, including cheeses, most fruit, and also quite good for wine. So, if you have access to space tempered in that range, or are willing to set up a small fridge for that, the problem solves itself.
I make spreadable-from-the-fridge butter in advance by adding grapeseed oil in a 1:1 ratio (recipe from here) and blending in a food processor for a smooth consistency. Pretty much any oil can work for this, but a neutral-flavored oil will keep the mixture tasting like butter.
I don't recommend using this for anything besides buttering bread; changing the consistency in this way can affect baked goods.
You can make your own spreadable-from-the-fridge butter but cutting it with an oil-water emulsion. Butter is about 75% fat in 25% water. So if you make a similar emulsion from, say, safflower oil (which is very light and flavorless) and water, you can add that to your butter, mix while soft, and re-refrigerate.
I personally use a 2-1 mix of butter and oil-water emulsion, I add a pinch of soy lecithin to the latter to stabilize it, and I add a bit of salt substitute (I have a medical condition that forces me to cut down on sodium). That way, the mix is salted to taste and spreadable right out of the container, and it saves a bit of money as well (safflower oil is cheaper than butter), and most people can't tell the difference. Of course you could use a more flavorful oil as well if you wanted that.
I usually let my butter outside the fridge, but in the winter it becomes solid and "non-spreadable"
What I do is turn on the light inside the oven and let it sit there for a few minutes - the same way some people do to rise dough. When I'm in a hurry I use a vegetable peeler and "peel" some thin layer of butter and put in the bread.
Here's how I soften a block of butter. I let two blocks of butter come to room temperature. then place them in a blender with a teaspoon of sunflower oil and 1/4 teaspoon of salt. I blend it slowly until the oil and salt are blended into the butter, then scoop it out, place it into an old clean butter tub, and refrigerate it. It does not go hard, even when cold it stays spreadable. Feel free to use a little more or little less oil depending on your own preferred consistency.