Place bittersweet chocolate and butter in a medium glass mixing bowl. Microwave for 30 seconds then stir. Microwave for another 30 seconds then stir.

Heat heavy cream and corn syrup in a small saucepan over medium heat until just simmering. Pour mixture over melted chocolate and let stand for 2 minutes.

Use a rubber spatula to stir gently until all chocolate is melted and mixture is smooth and creamy. Be careful to not incorporate air into the mixture by stirring too fast.

Add Madeira and gently stir to combine. Pour mixture into an 8- X 8-inch glass baking dish and chill for 1 hour.

Use a melon baller to scoop chocolate onto a sheet pan lined with parchment paper. Chill for 30 minutes.

Shape scooped chocolate into balls by rolling between the palms of your hands. Wearing latex or vinyl gloves in this process recommended. Roll balls quickly to keep from melting.

Wipe chocolate from your hands or gloves if there is residual chocolate on your hands and the balls slip around while rolling.

Place sweet ground cocoa and/or nuts into a dish or bowl (separate dishes or bowls if using both).

Place rolled truffles, one at a time, into the dish with either ground cocoa or nuts. Move the truffle around to coat.

Place coated truffles in an airtight container and chill.

Set out truffles for about 30 minutes to bring to room temperature prior to serving.

The truffles coated in ground cocoa may soak in the cocoa. If so, you can re-coat with additional ground cocoa.