Melt butter in a microwave then add the caster sugar and cocoa powder to the melted butter. Heat in the microwave in 30 second bursts until the sugar has almost dissolved and the ingredients have combined, stirring after each burst.
Allow the mixture to cool slightly before adding the vanilla extract, eggs, red food colouring, pinch of salt and white vinegar. Whisk until just combined.
Sift in the flour and gently fold until just combined.
Pour the brownie mixture into the prepared baking tin. Bake red velvet brownies in the preheated oven until cooked through. Leave in the tin to cool completely.