To begin, preheat your oven to 180°C (fan 160°C). Line two 18cm round cake tins with parchment paper. Lightly grease the sides with butter. Move on to mixing the batter.

In a large bowl, beat eggs and sugar until pale and airy. Stir in sour cream and vanilla extract. In a small bowl, combine baking soda with vinegar, then quickly stir it into the egg mixture. Transition to adding flour.

Gradually fold in the flour with a spatula until the batter is smooth and lump free. Do not overmix, as this may toughen the crumb. Move to baking.

Divide the batter evenly between the tins. Bake for 20 to 25 minutes until golden and a skewer comes out clean. Let cool in the tins for 10 minutes before transferring to a rack. Move to frosting preparation.

In a medium bowl, whisk together sour cream, icing sugar, and vanilla extract until smooth and slightly thickened. Chill for 10 minutes before use. Proceed to layering.

Once the sponge layers are fully cool, slice each in half horizontally to create four thin layers. Trim any domed tops and save the offcuts for garnish. Begin assembly.

Place the first sponge layer on a serving plate. Spread a generous spoonful of sour cream frosting evenly on top. Repeat with remaining layers, ending with frosting on top. Proceed to finishing the cake.

Frost the sides of the cake with remaining sour cream. Crush the sponge offcuts into crumbs and press them gently onto the top or sides. Add crushed walnuts or chocolate shavings if using. Continue to chilling.

Refrigerate the assembled cake for at least 6 hours or overnight. This allows the cream to soak into the sponge and develop its signature soft, melt-in-the-mouth texture. Move to serving.

Slice gently using a sharp knife. Serve cold or slightly chilled. Pair with strong black tea or coffee. Presentation tip: A dusting of icing sugar just before serving adds a soft finish.