Whisk dry ingredients (flour and cornstarch) together and set aside.
Cream butter and sugars with paddle attachment, on medium-low speed, until fluffy and pale in color (about 5 minutes).
Add egg and extract and mix on low speed until thoroughly combined. Scrape down the paddle with a spatula, and then continue to mix on low speed for another 30 seconds to ensure everything is fully incorporated.
Add dry ingredients, all at once, and mix on low speed just until dough starts to come together and no dry bits remain. Do not overmix.
Divide dough and dye with gel food coloring. Be careful not to overwork your dough. You can also make several batches of dough, and add food coloring when you add your eggs.
Turn out dough onto cling wrap, wrap tightly, and refrigerate overnight (or at least 4 hours).
Knead, roll, cut out center designs, stack together, and return freezer until firm.
Fill indentations around frozen center design with dough color of your choice. If dough becomes too soft, return to freezer.
Continue adding dough around center design until it is fully surrounded. Roll log to smooth out, roll in sprinkles or sanding sugar, tightly wrap in cling wrap, and return to freezer until firm.
Slice ¼" slices from frozen log, and place on silicone lines baking sheets 1" apart.
Bake at 350° for ~15 minutes, turn pan, and bake for another 1-5 minutes until edges are starting to turn a golden brown.
Remove from oven and allow to fully cool on cookie sheet before handling.
Cookies will stay fresh at room temperature in an airtight container for up to 1 week.