Place the butter into the bowl of the stand mixer (or a large bowl if using a hand mixer). Beat butter until light and fluffy, about 1 minute. Add in brown sugar and granulated sugar and beat until well creamed. Beat in the egg, vanilla extract, salt, and buttermilk. Finish the wet ingredients by adding in the food coloring and mix. Once combined, set aside.
In a separate bowl, mix together flour, baking powder, cornstarch, and cocoa powder. Once combined, slowly incorporate the dry ingredients into the wet ingredients. If the dough is not as red as you would like, feel free to add in more food coloring until it’s your desired color.
Add in white chocolate chips and mix until combined. Chill the dough in the fridge for at least 30 minutes. Preheat the oven to 350°F while dough is chilling.
Use a small cookie scoop to make sure there is the same amount in each cavity (or you can weigh the dough on a scale before putting it in the cavity). Each cookie should weigh 23g. Press the dough into a flat layer in the silicone mold. If you would like pretty, white chips on the top of each cookie, keep a few chocolate chips aside and place on top of each cookie before baking.
Bake for 10-12 minutes, or until slightly soft to the touch but not gooey (because of the color, red velvet cookies can be hard to tell if they’re “golden brown”, thus the touch method is necessary for determining "doneness.").