To make centers, simply combine cream, butter and corn syrup in a small saucepan and bring to a simmer over low heat. Remove pan from heat and let cool for 5 minutes.
Melt chocolate by placing chopped chocolate in the top of a double boiler over a pan of simmering water. Stir with a rubber spatula as chocolate melts. The chocolate will melt quickly. When chocolate is almost all melted, remove from simmering water, continuing to stir until it has melted completely.
Slowly pour cream mixture into melted chocolate, whisking until blended and smooth.
Let center mixture sit out for 2 hours on a counter.
Before you begin to pipe centers, prepare the pasty bag fitted with the tip and the jellyroll pan or baking sheet.
With an electric mixer on medium speed, whip chocolate mixture until it lightens in color, about 1 minute. You will see a definite change in color. Spoon chocolate mixture into the pastry bag. Roll up open end of pastry bag until your fingers are pressing against the chocolate. Hold pastry bag upright and squeeze a 3/4" ball straight down onto the jellyroll pan. Space balls about 1-2" apart. Repeat until all chocolate has been used. Refrigerate and chill for at least 1 hour.
Melt 12 ounces of chocolate just as before. Remove centers from refrigerator and have ready.
Sift cocoa into a small bowl and have ready.
Allow melted chocolate to cool to 90 degrees F. Place a center on a chocolate dipping fork or a fondue fork and dip center in melted chocolate until completely coated. Drop coated truffle into bowl of sifted cocoa. Repeat for all centers. Gently toss truffles in cocoa until completely coated and roll them in a strainer or on wax paper to remove excess cocoa. Resift leftover cocoa and you can re-use it.