Put the coffee in a small custard cup or ramekin. Pour the boiling water out of the kettle into your measuring spoon, then immediately pour it over the coffee. Stir with a spoon to dissolve the coffee. Cool for 5 minutes.
In a bowl, cream the butter with the vanilla extract. Blend in the dissolved coffee.
In another bowl, sift together the remaining ingredients (except optional confectioners' sugar). Blend the dry mix, about one-third at a time, into the butter until evenly mixed.
Divide the dough in half. Place each half on a sheet of wax paper and shape into cylinders about 4 cm in diameter. Wrap them up in the wax paper and refrigerate for at least 2 hours, or up to 2 days. You can also overwrap the dough in a plastic bag and freeze for up to 2 months; thaw overnight in the refrigerator.
When you're ready to bake, preheat the oven to 160ºC . Using a serrated knife, slice the cookies about quarter to 1 cm thick and place them on ungreased baking sheets. Bake for 25 minutes, or until the tops feel firm to the touch.
Transfer the cookies to a rack and cool to room temperature. Dredge the cooled cookies in confectioners' sugar if you like. Store in an airtight container.