Tiramisu + chocolate sponge cake = a dessert that everyone's going to love.
Start by preheating the oven to 175 C. Line your square tin with baking paper.
In a large bowl, combine all of the dry ingredients.
Add all of the wet ingredients, and carefully mix through. It will be quite a thin, pourable batter. Pour into the lined tin, and pop into the oven to bake until a toothpick comes out mostly clean with just a couple crumbs attached, around 40 minutes.
While the cake bakes, prepare the syrup. Combine sugar and water in a saucepan, stir, and bring to a boil. Turn the heat down to low, then let it cook for a further few minutes. Turn off the heat, then stir through the rum. Let cool.
Once the cake is out of the oven, let it cool for at least 15 minutes in the tin before transferring to a wire rack. Poke holes with a fork or toothpick all over the cake, and generously drizzle with the syrup.
Allow the cake to cool. Meanwhile, prepare the cream. Combine mascarpone, double cream, powdered sugar, and vanilla. Beat with an electric whisk until combined and a nice spreadable consistency.
Dollop the cream onto the cake, and finish with a dusting of cocoa and espresso powder. Enjoy!