Step 1Put the chopped chocolate in a heatproof bowl.
Step 2Bring the heavy cream to a boil, then pour it over the chocolate. Working with a whisk or spatula, gently stir the chocolate and cream together. When the ganache is smooth, 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.