Preheat oven to 375⁰F. Butter the sides of a 7 to 8-inch spring form pan and line bottom with a round of parchment paper.
Melt butter in microwave in a 2-quart batter bowl. Blend cocoa and sugar into melted butter.
Whisk in eggs and vanilla. Pour batter into prepared pan.
Bake 25 to 30 minutes or until center tests done.
Allow cake to cool. Remove side ring from pan. Transfer cake to serving plate and dust with powdered sugar. Slice in thin slices.