Preheat your oven to 180°C/160°C fan/gas 4. Grease and line a 30x23cm (12x9 inch) rectangular baking tin with baking parchment.

In a large mixing bowl, cream together the softened butter and caster sugar until light and fluffy. Beat in the eggs one at a time, adding a tablespoon of flour with each egg to prevent curdling. Sift in the remaining flour, cocoa powder and baking powder, then fold gently until combined. Stir in the milk and vanilla extract to create a smooth, pourable batter.

Pour the batter into your prepared tin and spread evenly. Bake for 25-30 minutes until the cake is springy to the touch and a skewer inserted into the centre comes out clean. Allow to cool completely in the tin.

While the cake cools, melt the chocolate, butter and golden syrup together in a heatproof bowl set over a pan of simmering water (or microwave in 30-second bursts). Once melted and smooth, remove from heat and sift in the icing sugar, stirring until you have a glossy, pourable icing.

Pour the warm icing over the cooled cake and spread evenly with a palette knife. Add sprinkles or chocolate chips if using. Leave to set for at least 30 minutes before cutting into squares.