Heat the oven to 180C/350F/gas mark 4. Grease a 23cm square cake tin and line the base with a strip of baking parchment.
Sift together the flour, cornflour, baking powder and salt.
Whisk together the butter and sugar with an electric mixer for a good few minutes until extremely light and fluffy. Whisk in the egg yolks one at a time, whisking well after each addition. Whisk in the vanilla extract.
Fold in one third of the flour, then half of the milk followed by another third of flour, the rest of the milk then the rest of the flour.
Scrape the batter into the prepared tin and spread it level. Bake for about 40 minutes, until a skewer inserted into the centre comes out clean. Leave the cake to cool in the tin for 20 minutes then carefully turn it out onto a wire rack and leave to cool completely.
Once the cake has cooled make the ganache. Place the chopped chocolates and the cream in a bowl set over a pan of gently simmering water, do not let the base of the bowl touch the water. Stir regularly until melted and smooth.
Remove from the heat and add the golden syrup and butter; stir until melted then place in the fridge until cold but not set.
Whisk with an electric mixer until thickened, be careful not to overmix. If you find that it is not becoming thick then the ganache may be too warm still, pop it back in the fridge until cool then try again.
Spread the whipped ganache over the cake and sprinkle with M&M's, mini M&M's, sprinkles, chopped chocolate bars etc.