Pre-heat oven to 180°C / 160°C fan assisted /gas mark 4 / 350°F and line and grease 2 x 8 inch (20cm) round 4 inch (10cm) deep cake tins.
Melt the chocolate in a bain marie, or a glass bowl set over a pan of boiling water, then set aside.
Pour the milk into a jug and stir in the apple cider vinegar. Let stand for 5-10 minutes to curdle slightly.
In a large mixing bowl, or stand mixer, whisk together the flours and starches, cocoa powder, espresso powder, bicarbonate of soda and salt until well mixed.
In a separate bowl mix together the vinegar milk, sugars, olive oil, eggs and vanilla then beat into the flour mixture.
Finally mix in the melted chocolate until completely incorporated.
Divide the mixture between the 2 cake tins and bake in the oven for 35 minutes or until an inserted skewer is removed without a wet crumb.
Turn out the cakes and cool on wire racks before filling with buttercream.