Preheat oven to 350° F. Grease a 9 x 13-inch baking pan.
Beat butter, sugar and brown sugar in a large bowl with an electric mixer until light and fluffy, about 3 minutes. Add eggs, one at a time, mixing well after each addition. Beat in melted chocolate and vanilla.
Stir together flour, baking soda, cream of tartar and salt in a medium bowl. Add to brownie batter in three additions, alternating with buttermilk. Mix gently after each addition. Do not over mix.
Pour batter into prepared pan and bake for 30 to 40 minutes, until a knife inserted in the center of the cake comes out clean.
Mix together chocolate chips and heavy cream and melt in a microwave or over low heat. Stir. Spread in a thin layer over brownies. Dust with powdered sugar. Cut into squares when cool.
HELPING HANDS: Kids can help make the frosting and spread it on the brownies.