1/2 c. butter
3 2/3 c. powdered sugar
1/2 c. cocoa
1 tsp. vanilla
1/3 c. milk

Microwave butter in glass bowl on high (full power) until melted, about 1 minute. Stir in cocoa until smooth. Microwave on high (full power) until mixture boils, 1/2 to 1 minute. Add powdered sugar and milk. Beat with mixer until spreading consistency. Blend in vanilla. Spread while warm.