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Preheat the oven to 350 degrees and line a large baking sheet with parchment paper. Set aside.
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In a large bowl with hand mixer (or with a standing mixer), cream butter, granulated sugar, and brown sugar together until light and fluffy.
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Add in the egg and egg yolk, and mix until combined.
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Next, add in vanilla extract, white vinegar, and red food coloring. Mix until combined and the red food coloring is well distributed.**
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Add in the flour, cornstarch, cocoa powder, salt, baking powder, and baking soda. Mix until well combined. (The dough will be very thick.)
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Roll cookie dough into about 1 - 1 1/2 Tbsp balls (you can use a cookie scoop or your hands) and place them evenly apart on the cookie sheet.
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Gently flatten the tops of the cookie dough, if desired. This ensures they bake with a flat surface, which makes frosting the cookies easier. However, they will cook fine if you choose not to flatten them.
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Bake for 9-10 minutes, or until the edges just look set (they will continue to bake as they cool on the pan so we don’t want to over bake them).
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Allow to cool on the baking sheet for 5-10 minutes, then transfer to a wire rack to finish cooling.