Step 1Heat the oven to 350 degrees. Butter a 9-inch-square baking dish.
Step 2Combine the stick of butter with the chocolate and espresso powder in the top of a double boiler or in a steel bowl set over simmering water in a pot. Heat, stirring often, until the butter and chocolate are melted. Remove from the heat and stir in the Kahlua, vanilla and sugar, mixing well. Let cool 5 minutes.
Step 3Stir the eggs into the batter until completely incorporated. Combine the flour, baking powder and salt and stir into the batter, mixing well. Stir in the nuts. Scrape the batter into the prepared pan.
Step 4Bake for about 25 minutes, until a toothpick inserted halfway between the edge and the center of the pan comes out clean. Do not overbake. Cool on a rack before cutting into bars.