Preheat the oven to 190°C/375°F (170°C fan) and grease and line a 20cm round cake tin with baking paper.
Soften the butter in the Thermomix bowl for 30 sec/speed 5 until creamy.
Add the remaining ingredients to the Thermomix bowl and mix for 10 sec/speed 5. Scrape down the bowl and along the bottom of the mixing bowl with a spatula then mix for another 5-10 sec/speed 5 until smooth.
Spoon the batter into the cake tin and bake for 35-40 minutes until firm to touch and a toothpick inserted into the middle comes out clean.
Cool for 10 minutes in the tin then turn out onto a wire rack to cool completely before icing.
Chocolate Buttercream
Add the butter to the Thermomix bowl and mix for 20 sec/speed 5 until creamy.
Insert the butterfly whisk, add the icing sugar and cocoa powder and beat for 20 sec/speed 3. Scrape down bowl.
Add the 20g milk and beat again for 20 sec/speed 2.5. Scrape down the sides and bottom of the bowl with the spatula and repeat for 10 sec/speed 2.5 until smooth.
Notes
Storage: Store the cake in an airtight container at room temperature for up to 3 days. If your kitchen is warm, it’s best to keep it in the fridge. If refrigerated, bring to room temperature before serving for the best texture.