Make the buttermilk by stirring the lemon juice into the milk and set aside.

Pre heat the oven to 180C/ 160C fan. Grease and line two 2 x 20cm cake tins.

Sieve the flour, cocoa, bicarbonate of soda, baking powder, and salt into a large mixing bowl and mix well.

In a second large bowl, whisk the buttermilk, oil, sugar, eggs and vanilla until well combined.

Gradually fold the wet ingredients into the dry until the mixture is just combined. Evenly divide the batter between both tins.

Bake for 30 minutes until well risen and a skewer inserted in the middle comes away clean.

Remove from the oven and leave to cool in the tins for 10 minutes, then turn onto a wire rack and leave to cool completely before icing.

To prepare the filling, add 160g raspberries (leave the remaining berries to decorate the cake), the lemon juice, water, sugar and cornstarch to a small saucepan. Bring to a gentle simmer and stir occasionally for 5 minutes or until the raspberries have softened and the sauce has thickened. Turn off the heat and allow to cool completely.

To prepare the buttercream, use an electric whisk to beat the butter until light and creamy. Gradually sift in the icing sugar, beating between each addition. Sift in the cocoa powder, add the vanilla and 2 tbsp milk and beat again to combine. Add the further tbsp of milk if needed to make the icing fluffy and spreadable.

Pipe a tall border of buttercream around the edges of one cake. Then spread over 2 tbsp of buttercream to create a light filling. Spread the raspberry filling over the centre of the cake, leaving the border of buttercream free. Sandwich over the second cake. Ice the top and sides of the cake with the remaining buttercream. Scatter over the remaining berries and chocolate shavings.