Chop the milk chocolate and semisweet chocolate up and place in a microwave safe bowl.
Melt chocolate in microwave, running the microwave in 20 second intervals and stirring the chocolate between each interval until fully melted. Allow to cool slightly.
In another small bowl, mix together cocoa powder and boiling water into a thick paste. Set aside.
In yet another bowl (Martha Stewart seems to be a big fan of dirtying as many bowls as possible), beat together powdered sugar and butter until smooth. Dump in chocolate paste, beat until combined. Pour in melted chocolate and beat to combine.