Preheat the oven and prepare the tin: Set your oven to 180°C (160°C fan). Grease a 20cm round cake tin and line the base with a circle of parchment paper. The paper should sit flat with no wrinkles — this ensures the cake releases cleanly later.

Combine the dry ingredients: In a large mixing bowl, sift together the flour, cocoa powder, baking powder, bicarbonate of soda, and salt. Sifting removes any lumps and aerates the mixture; you will see a fine, even, dark brown powder with no speckles of white flour remaining.

Add the sugar and wet ingredients: Stir in the caster sugar until evenly distributed. Make a well in the centre and pour in the oil, eggs, vanilla extract, and milk. Whisk gently at first to avoid a cloud of cocoa dust, then increase speed until the batter is smooth and glossy. It should have the consistency of thick double cream — it will drop from the whisk in a steady ribbon.

Fold in the raspberries: Gently fold half the raspberries into the batter using a silicone spatula. Do not overmix; you want some berries to remain whole so they burst during baking, creating little pockets of tangy jam. The batter will now have occasional crimson streaks — that is exactly what you want.

Bake the sponge: Pour the batter into the prepared tin and spread it evenly. Tap the tin once on the worktop to release any air bubbles. Bake for 25-28 minutes. You will know it is done when the top feels springy to a light touch, the edges have pulled away slightly from the tin, and a skewer inserted into the centre comes out with just a few moist crumbs attached — not wet batter.

Cool completely: Leave the cake in the tin for 10 minutes, then turn it out onto a wire cooling rack. Peel off the parchment paper and let it cool completely. You should hear a faint crackling sound as the cake settles — that is the steam escaping, which keeps the crumb tender.

Whip the cream and assemble: Just before serving, pour the double cream and icing sugar into a chilled bowl. Whisk until soft peaks form — when you lift the whisk, the cream should hold a gentle peak that flops over slightly. Spread the cream over the cooled cake, then scatter the remaining fresh raspberries and chocolate shavings on top. Serve immediately while the cream is cold and billowy against the rich, fudgy sponge.