In a large bowl, whisk together melted butter, sugar, cocoa powder, vanilla extract, red food coloring, white vinegar, eggs and salt.
Fold in the flour just until combined.
Alternately add scoops of the cookie batter and red velvet brownie batter to the prepared baking pan.
Bake the brookies until the cookie dough is golden and the batters are fully set, 30 to 40 minutes. (See Kelly’s Notes.) Remove them from the oven and immediately sprinkle them with the sea salt. Let the brookies cool completely in the pan before slicing and serving.