Make sure all of the ingredients are at room temperature before starting.

To make the chocolate cake

1. Preheat the oven to 180°C/350°F (without fan).
2. Line the bottom of a 23 cm/9 inch round cake pan and grease the sides.
3. Place all of the ingredients into the bowl of a food processor and blitz until the mixture is well-combined. The batter should be thick and smooth.

Alternatively, if you do not have a food processor:

1. Cream the butter and sugar in the bowl of a stand mixer with the flat paddle attachment.
2. Slowly beat in the flour, cocoa, sugar, baking powder, baking soda and salt.
3. Beat on low speed until all of the ingredients are incorporated.
4. In a small bowl or jug, lightly whisk together the sour cream, eggs and vanilla.
5. Slowly add the wet ingredients to the mixer, and beat until the batter is thoroughly mixed.

To Bake the Chocolate Cake

1. Pour the batter evenly into the prepared cake pan and bake for 35-40 minutes, or until a skewer inserted in the middle comes out clean.
2. Place the cake pan on a wire rack and let the cake cool in the pan for about 10 minutes, before turning it onto the wire rack to cool completely.

To make the chocolate frosting

1. Melt the chocolate and butter in a bain-marie or a bowl over a pan of simmering water, taking care not to let the bowl come into contact with the water. If you have an induction stove, you can melt the chocolate and butter directly in a saucepan over low heat.
2. Once the chocolate has melted, take the bowl off the heat.
3. Stir in the sour cream and vanilla extract.
4. Whisk the icing sugar (powdered sugar) into the chocolate mixture.
5. The frosting should be thick enough to cover the cake, but soft enough to spread easily.
6. Frost the cake as per the photo with a tall layer of frosting, or cover all sides of the cake completely with the frosting - however you like.
7. Leave the cake to set for at least a few hours before serving.