Preheat oven to 350 degrees

Butter a 9-inch-square baking pan. Cream the butter with ½ cup of the brown sugar until creamy. Beat in the egg yolks. Sift flour, baking powder and salt together. Fold this into butter mixture. Pat the dough into the prepared baking pan. Beat the egg whites until semi-stiff. Add the remaining brown sugar and beat until very stiff. Fold peanuts into the egg whites and spread the mixture over the dough. Bake for 35 minutes. When the cake has cooled, cut it into 36 squares and store them in an airtight container.