Line a 9- x 13-inch baking pan with parchment paper.

Combine the cream and corn syrup in a 2-quart saucepan and bring to a boil. Remove from the heat. Add the butter and chopped chocolate to the cream and stir until smooth and homogeneous to form the ganache.

Pour the finished ganache into the baking pan to make a thin layer and cover with plastic wrap. Refrigerate for 1 hour or more until the ganache is firm.

Put the ganache in a mixer bowl and mix on medium speed, using a paddle attachment, for 30 seconds. Or, stir vigorously in a mixing bowl by hand, using a spatula.

Allow the ganache to rest at room temperature for 5 minutes. Meanwhile, line a sheet pan with parchment paper, sift the cocoa into a bowl, and set aside for later use.

Using a teaspoon, scoop out balls of ganache and place on the sheet pan at room temperature. When all of the ganache has been scooped, roll each portion by hand into a ball and drop into the cocoa. Roll each truffle in cocoa to coat. Chill until firm.

Transfer the truffles to airtight storage containers and separate the layers with parchment or waxed paper.