Preheat oven to 350°F. Lightly grease a 9-inch cast iron skillet or cake pan with cooking spray. Set aside.
In a large bowl, beat the butter and sugar until light and fluffy, about 3-4 minutes, stopping to scrape sides of bowl. Add egg, food coloring, vinegar, and vanilla bean paste/extract. Beat until combined. Set aside.
In a medium bowl, whisk flour, cocoa powder, cornstarch, baking soda, and salt. Gradually add flour mixture to butter mixture, beating until just combined. Gently fold in mini chocolate chips.
Using lightly greased hands, press dough into prepared skillet/pan. Top with additional chocolate chips, if desired.
Bake for about 25 minutes, or until edges are firm. Remove from oven and serve warm with vanilla ice cream, if desired. If you would like to serve cookie in pie slices, let cool for at least 20 minutes before cutting.
See my tips and tricks for making this Red Velvet Skillet Cookie above recipe box.