Heat oven to 350 degrees F. Line 8×8-inch baking pan with parchment paper.

In large saucepan over low heat, melt butter. Remove from heat and whisk in sugar. Add one egg at a time, whisking each until incorporated, and finish by whisking in vanilla until smooth.

Stir in cacao powder, flour, salt and baking powder. Pour into prepared pan and spread out evenly.

Bake for 25 to 30 minutes, until batter is just set.

Let cool completely, then cut into desired size.