Empty contents of coffee bag into a small custard cup or ramekin. Pour 1 tablespoon boiling water into a 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 1 cup (2 sticks) unsalted butter (at room temperature) with 1 teaspoon vanilla extract. Blend in the dissolved coffee. Empty the contents of the jar into a bowl and mix to blend well. Blend the dry mix, about one-third at a time, into the butter-coffee mixture until evenly mixed. Divide the dough in half. Place each half on a sheet of wax paper and shape into cylinders about 1 1/2 inch in diameter. Wrap them in the wax paper and refrigerate for at least 2 hours, or up to 2 days.
When ready to bake, heat the oven to 325 degrees F.
Using a serrated knife, slice the cookies about 1/4 to 1/3 inch 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.
3These cookies are especially good with ice cream. They're delicately flavored with coffee and not too sweet!