01 -
Set your oven to 350°F (180°C, or 160°C if using a fan). Line up a square 9-inch pan with baking paper.
02 -
Pop the butter in the microwave and zap it in short bursts until melted. Stir it together with the sugars in a bowl so everything's smooth.
03 -
Toss in the eggs, the extra yolk, and vanilla. Mix gently without whipping it too much (this keeps the texture right). Pour in the oil and stir in the cocoa powder until just pulled together.
04 -
Scoop two dollops of batter into a small bowl. Stir through the red dye until even. Pour this mix back into the main bowl, blending it fully.
05 -
With a spoon or spatula, fold in the flour, cornstarch, and a pinch of salt. Get rid of any powdery bits. Stir the vinegar through. If you want chips, throw them in!
06 -
Spread the batter into your prepared tray and bake till a toothpick poked in comes out almost clean (20-25 minutes). Let it fully cool off before icing.
07 -
Beat softened butter with cream cheese till smooth. Gradually add icing sugar, then crank the speed to high for a fluffy finish. Stir in vanilla at the end.
08 -
Evenly spread the frosting over the cooled bake. Slice it up into squares for serving.