Roll the white fondant, medium brown fondant and dark brown fondant into 12-inch long ropes. Then combine into one rope and twist together. Roll and twist the rope together, folding the ends inwards after each roll. Repeat this process 6 to 8 more times until the mixture is a marbled look.
Once marbled, roll out the fondant into a 12-by-5-inch panel, dusting the counter with cornstarch if sticky. Use a butter knife to create the wood grain lines. Punch out 6 circles with a 1 1/2-inch biscuit cutter.
Roll the red fondant out with a little cornstarch until 1/4-inch thick. Use a number-shaped cutter to punch out 6 of your desired number (great for birthdays). Poke the numbers gently out from the cutter with the blunt edge of a skewer. Affix each number to the center of a wood grain circle with a little shortening. Place 1 circle on top of each cupcake.
Dry out the toppers in a 170 to 180 degree F oven for 10 minutes if they are too soft.