Prepare the oven and tins – Preheat the oven to 180°C (350°F). Grease and line two 8-inch cake tins.

Mix dry ingredients – Sift together self-raising flour and cocoa powder in a bowl to remove lumps.

Cream butter and sugar – Beat butter and caster sugar until pale and fluffy.

Add eggs gradually – Mix in the eggs one at a time, beating well after each addition.

Combine ingredients – Fold the flour mixture into the batter, adding milk and vanilla extract.

Bake the sponge layers – Divide the batter between the tins and bake for 20–25 minutes until the cakes spring back when touched.

Cool completely – Allow the cakes to cool on a rack before assembling.

Prepare the chocolate filling – Heat the cream until warm, pour over chopped dark chocolate, then stir until smooth. Add butter and mix well.

Assemble the cake – Spread chocolate filling over one sponge layer, then place the second layer on top.

Prepare the mirror glaze – Heat cream, sugar, and golden syrup in a saucepan. Add chocolate and stir until smooth. Dissolve gelatin in warm water and mix it into the glaze.

Glaze the cake – Place the cake on a rack and pour the glaze slowly over the top so it flows smoothly down the sides.

Allow glaze to set – Leave the cake at room temperature until the glaze becomes glossy and firm.