Step 1Put the chopped chocolate in a heatproof bowl.
Step 2Bring the heavy cream to a boil and pour over the chocolate. Working with a whisk or spatula, very gently stir the chocolate and cream together. When the ganache is smooth and shiny, stir in the butter.
Step 3Cool the ganache by putting the bowl in a larger bowl filled with ice cubes and cold water. Stir almost constantly, so it doesn't harden; when the ganache is thick enough to pipe and stay where it's piped, spoon it into the pastry bag and fill the macarons.