Melt chocolate in a microwave-safe bowl in 30-second intervals on High (100%), stirring between heating. Do not overheat. The bowl should be almost cool to the touch. Overheating will quickly cause the chocolate to burn.
Heat corn syrup in a separate microwave-safe bowl on High (100%) for 45 seconds.
Pour syrup into melted chocolate and stir with a rubber spatula until completely blended, about 30 generous folds of the spatula. Make sure the spatula and bowl are scraped clean of white chocolate. If any white chocolate has not made contact with corn syrup, you will have lumps in your finished modeling chocolate. Watch for white chocolate streaks while you stir. Mix until the streaks are gone.
Do not over-mix. If you over-mix, you will notice your chocolate becoming crumbly and falling apart. The over-mixing is pulling out too much cocoa butter (fat) from the chocolate. The crumbly chocolate will settle at the bottom of the bowl and the clear liquid fat will rise to the top. You will not be able to blend the fat back into the chocolate once this happens.
Line a quarter-sheet (9 x 13 in or 23 x 33 cm) cake pan with a large sheet of plastic wrap, making sure there is plenty of overhang. Pour chocolate mixture into pan and wrap the over-hanging plastic wrap around the chocolate. Pull the plastic wrap straight and tight over chocolate. If the plastic gathers and pokes into the chocolate, you will have trouble pulling it out of the modeling chocolate after it hardens. Cover tightly with plastic wrap and set aside at room temperature overnight.
The next day, cut the block of modeling chocolate into knead-able sections. The block will be greasy. It needs to be kneaded. Dust a little cornstarch on the table to knead if chocolate is sticky. If the block is too hard for you to knead, heat in microwave 5 seconds at a time until you can begin to easily press your fingers into the block with less force. You will notice the block changing into a silky and pliable clay-like ball. It will be a light ivory color after it is kneaded. This is now Modeling Chocolate.
Cover with plastic wrap (Press-n-Seal is best) and drop in a resealable plastic bag for storage.
Store at room temperature for 3-4 months before the oils begin to dry out, or in the freezer for up to 2 years.
To color: Knead in gel paste or liquid paste color of any kind. The water-based icing colors will be perfect for the job. Start with a small amount, and add a little bit at a time until you get the color you want.
To make white, you must use white (titanium dioxide) powder or liquid color made for chocolate. Add it to the chocolate a little bit at a time until you get the color you want. The water-based white colors are too liquid for this job.
You can use pre-colored candy melts to make modeling chocolate, which is a helpful tip when you need several pounds of one color. You can also pre-color your melted white chocolate before adding your corn syrup.
Tip: If you're getting chunks of white chocolate in your finished product, then you did not blend the corn syrup with the melted white chocolate completely. You can try to re-melt in the microwave and mix again by getting it to a very soft and pliable temperature. But do not overheat or the fat will separate from the chocolate.
If you're getting grit in your finished product, then you have over-mixed and pulled too much fat out of the white chocolate. The modeling chocolate will be a crumbly mess as you knead. There is no way to restore it to a smooth and workable clay at this point.
The same recipe, minus 1/4 cup (60 ml) corn syrup, works for milk/dark chocolate and does not have as many issues as white chocolate. However, you're stuck with brown and have no color options other than black.