Preheat the oven to 350F (180C). Lightly grease a 9x13 inch (23x33 cm) pan. Alternatively, line with parchment paper so that there's an overhang around the edges.
In a medium bowl, whisk together the flour, cocoa, baking powder, baking soda and salt.
In a large bowl, beat together the butter, granulated sugar and brown sugar until creamed.
Beat in the egg, egg yolk, vanilla, red food coloring and vinegar into the butter mixture until you no longer see lumps of butter.
Add the dry ingredients into the butter mixture (if lumpy, sift the dry ingredients into the bowl). Then mix together, starting with the mixer on a low speed. Mix together until you no longer see lumps or streaks of dry ingredients.
Spoon the dough into the prepared pan and spread/press into an even layer. If you plan to press it down, use a piece of wax paper in between your hands and the dough so that the dough doesn't stick to your palms.
Bake in the middle of the preheated oven for about 25-30 minutes, or until the top looks set and the edges are starting to pull away from the sides of the pan.
Cool the bars fully before frosting.