Preheat the oven to 180°C, fan 160°C, gas 4. Beat the butter with the sugar until light and creamy. Add the eggs, one at a time, mixing well. Mix in the flour, vanilla and 100ml milk until smooth. Spoon half the mixture into another bowl and stir in the cocoa and 1 tablespoon milk.
Grease a 20cm cake tin and dust with flour. Place alternate spoonfuls of each mixture into the cake tin. Drag through a skewer to create a marbled effect. Bake for 30 minutes, then cover with foil and bake for a further 10-15 minutes, or until a skewer inserted comes out clean. Leave to cool for 10 minutes.
For the ganache, heat the syrup, sugar and 2 tablespoons water in a pan, stirring. Bring up to the boil, then remove from the heat. Add the chocolate. Leave to melt, then stir until glossy. Spread on the cake and top with the crispies.
Cook's tip: if you don't have chocolate wheat crispies to hand, you can top with any kind of chopped nuts or a few shavings of chocolate.