It's best to use store-bought ground almonds (if you're in the US, use blanched almond flour - the fine grainy kind (NOT the powder kind that's like regular flour)) - for this recipe. Please don't try using home-ground almonds, as grinding them yourself can result in ground almonds that are a little too fine, and slightly oily for the recipe - this can cause the amaretti to go flat. I've tested this recipe with both regular UK ground almonds, and with blanched almond flour that I picked up in the US last time I was there. When testing the US almond flour, I used Full Circle Market Almond flour, and this produced the same results as the regular UK ground almonds.