Melt white chocolate in a microwave-safe glass or ceramic bowl in 30-second intervals, stirring after each interval, 1 to 3 minutes.
Heat corn syrup until almost as warm as the melted chocolate and combine with chocolate; it may begin to harden and be difficult to stir, but be sure to fully incorporate. Let rest in a cool place, such as a refrigerator or countertop, until very stiff, about 2 hours.
Break off a piece in the size you need and begin to play with it; the warmth of your hands will make it a Play-Doh consistency. Mix in gel food coloring as desired.
Dust the countertop and a rolling pin with cornstarch and place a ball of modeling chocolate on top; roll to desired size. To cover a cake, roll the chocolate up on the rolling pin for easier transfer to the cake.