Is there any way to dissolve coffee grinds once added to a butter cream frosting?
I used instant coffee crystals in the past and they have always dissolved when simply added to the butter cream during the mixing stage — without having to dissolve in any sort of hot liquid prior to adding.
I have made this mocha butter cream frosting several times in the past, but didn’t think it through when I decided to use regular coffee instead of instant, which I discovered — albeit too late — that I was out of.
It's a bit late now, but if you've only got real coffee and want to make buttercream you can still do it. I made a latte buttercream by brewing very strong coffee in hot milk, then straining it and adding the liquid to the plain buttercream ingredients, tasting and adjusting the proportions for texture. It worked well.
I did this by:
Brewing 1 Tbsp of ground coffee in 3–4 Tbsp of hot milk for a few minutes and straining,
Beating together 250 g slightly softened butter and 500 g icing sugar, adding most of the latte.
It must have been about a year ago when I did this – they were pumpkin spice latte cupcakes because of a discussion caused by the drinks of the same name, but I've found my notes with the real proportions.
The particle size of regular ground coffee is too large to use in icing, as you’ve found; you’ll be able to see and feel the individual particles. But coffee specifically ground for making espresso is much finer — it’s much closer to a fine powder than “grounds” — works nicely. It still won’t actually dissolve, per se, but the individual bits are undetectable. Tastes better than instant, too.
As discussed here, the human palate is remarkably sensitive to granularity. Even particles as fine as 2 microns have an effect on the subjective perception of food. This means it's going to be very hard to grind something fine enough by hand that it doesn't significantly affect the finished product.