Preheat oven to 350°F / 175°C. Line two 8" square spring-form cake tins with greaseproof paper.
Cream butter and sugar together until light in colour. This is far easier if the butter is warm to start with.
120 g unsalted butter, 300 g caster sugar
Add eggs one at a time, beating well after each addition to combine well.
4 large eggs
Add 2 teaspoons of vinegar to the milk. Allow to sour for 10 minutes to produce buttermilk.
25 ml white vinegar, 250 ml full fat milk
In a separate bowl, mix together the oil, cocoa powder, red food colouring and vanilla extract until smooth.
75 ml olive oil, 10 g cocoa powder, 45 ml red food colouring, 2 tsp vanilla extract
Stir the coloured mixture and the remaining 20ml vinegar into the creamed sugar mixture to combine.
25 ml white vinegar
Sift the flour, bicarbonate of soda and salt together in a separate bowl.
350 g plain flour, 2 tsp bicarbonate of soda, 2 tsp salt
Add the flour mixture and the buttermilk alternately spoon by spoon to the wet mixture, mixing well in between.
Add smooth batter to the two tins. Bake for 25-30 minutes, or until a skewer inserted into the centre of the cake comes out clean. Cool completely on a wire rack before adding frosting.