Preheat oven to 350 degrees F (175 degrees C). Grease a 9 x 13 inch pan.
Melt the chocolate and butter in the top of a double-boiler, stirring sporadically. When melted and combined, add the sugar and eggs, and stir until combined. Sift the flour, salt, and baking powder into the pan; stir until combined. Stir in the nuts (or raisins, or M&M candies, or toffee bits, etc.).
Spread the batter in the pan, smoothing it into the corners.
Bake for about 35 minutes, but don't let it over-bake. Cool before cutting.