Preheat the oven to 350°F. Grease a 9x9-inch baking pan or line it with parchment paper.
Mix the flour, baking powder, salt, and 4 tablespoons of cocoa powder in a medium bowl.
Cream the margarine and the sugar in a separate bowl until light and fluffy. Add the eggs and beat lightly, one egg at a time, into the cream.
Gradually fold the dry ingredients into the wet mixture using a spatula or wooden spoon. Mix until just combined, being careful not to overmix.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs.
Allow the brownies to cool completely in the pan before icing.