Preheat oven to 180°C (350°F). Grease or line a small rectangular cake pan (about 8x8 inches).
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a separate cup, stir vinegar into milk and let sit for 5–10 minutes to create buttermilk (or use 60g buttermilk directly).
Using an electric mixer, beat the egg and granulated sugar for 2 minutes until light and thickened.
Slowly add the olive oil while mixing, then add the dry ingredients alternately with the milk mixture. Mix until just combined.
Pour the batter into the prepared pan and smooth the top.
Bake for 30 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack and cool completely, about 2 hours.