Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).

Cream butter and sugar until combined. Lower mixer speed and add remaining ingredients slowly, in the order they appear, just until a dough forms. Turn off mixer.

Divide dough in half. Form two logs and place them on a lined baking sheet. Bake for half an hour. Allow to cool for about 15 minutes, then slice with a sharp knife into rectangular pieces, on a slight angle.

Lower oven temperature to 325 degrees Fahrenheit (170 degrees Celsius). Place cookies on their side and bake for another seven minutes, then turn them over and bake for another five to seven minutes, until toasted through. Cookies will harden more as they cool.

Mix all ingredients together in a bowl. Keep mixing every so often to prevent the mixture from hardening.

Drizzle over biscotti.