There are two things happening when you make espresso:
The pressure of the espresso machine pushes the water through the grounds faster than most other coffee-making methods. In general, the good flavours in coffee are highly water-soluble and the bitter, acidic parts are less water-soluble, so by pushing the water through the grounds quickly, you get proportionally more of the good stuff and less of the bitter.
More significantly, by grinding the coffee very finely and packing it in (tamping), the water doesn't just flow past the coffee particles, the pressure forces the water through the particle where it pushes out some of the aromatic oils which form an emulsion in the liquid.
That emulsion - the crema - is what makes espresso different from coffee, and you need both a fine grind and tamping to get it.
If you're really keen on getting this right, you could start by checking your temperature and pressure - you want ~200 F and ~9 bars.
Your extraction time (30s) is good, and so is going for a double shot, but I'm not sure what you mean by "always go over." If you're getting good crema, it should ooze out, almost like honey or syrup, and you should stop the extraction when it starts to turn whitish. If that happens in less than 30s, you should grind your coffee a bit finer and try again. If it takes more than 30s, you need to go a bit coarser.
If you still find the espresso tastes burnt, try different beans.