Heat the oven to 180C/160C fan/gas 4. Oil a 22 x 32cm cake tin, baking tin or casserole dish and line with baking parchment.

Sift the flour, cornflour, bicarb and ½ tsp salt in a medium bowl. Set aside. Combine the cocoa powder and the cooled espresso in a jug. Set aside.

Use a hand mixer on a medium speed to cream together the vegetable fat and sugars for 2-3 mins, or until completely combined and lighter in colour. Alternatively, use a stand mixer fitted with a paddle attachment to do this.

Add the eggs, one at a time, followed by the vanilla, and mix for a further 2-3 mins, or until fluffy, scraping down the sides of the bowl to ensure everything is properly incorporated. Add the cocoa powder and coffee mixture. Mix to combine.

Add your dry ingredients in batches alternating with the buttermilk, mixing well after each addition, ending with the dry ingredients. Fully combine everything.

Pour the batter into the prepared pan, tilting the pan gently to spread the batter around evenly. Bake for 30-35 mins or until a tester inserted in the centre of the cake comes out cleanly. Remove it from the oven and leave to cool for 30 mins in its tin on a wire rack, then transfer to the freezer for at least 1 hr.

While your cake is cooling, make the buttercream. Use an electric whisk to cream together the fats and icing sugar for 2-3 mins, or until light and fluffy. Alternatively, use a stand mixer fitted with a paddle attachment to do this. Add the vanilla, a pinch of salt and double cream. Mix for 30 seconds or until well-blended.

Transfer the buttercream to a piping bag fitted with a very large (approximately 13mm) round tip. Chill until needed.

When you’re ready to add the bumps to your cake, remove the buttercream from the fridge and the cake from the freezer. Pipe the buttercream across the frozen cake width-wise in 6-7 evenly spaced lines about 2.5cm apart.

Return the bumpy cake to the freezer for at least 30 mins.

Meanwhile, make the fudge frosting. Combine the sugar, 250ml water, liquid glucose or corn syrup, cocoa powder and ¼ tsp salt in a medium-large saucepan. Cook over a low heat, stirring constantly until the sugar dissolves and everything mixes together. Add the butter and vanilla, then increase the heat to medium-high and bring the mixture to the boil. Once the mixture begins to boil, do not stir it, however much you’re tempted to.

Cook the bubbling sugar mixture until it just reaches 112C on a sugar or instant thermometer; the bubbles will be quite big at this stage and will be popping with a cracking sound. Turn off the heat. Dip a spoon into your mixture to test its consistency. The mix should thickly coat the spoon and drizzle off it in ribbons, rather than in a thin stream. If the frosting is too thin, return the pan to the heat, bring to the boil and cook for a further 1-3 mins, dipping your spoon in it to test the consistency every 30 seconds. When it's the right consistency, remove the pan from the heat immediately.

Let the frosting cool slightly for around 3-5 mins. The mix will be very shiny. You still want it to be warm and glossy when you pour it over your cake.

While the frosting is cooling, remove the cake from the freezer, then pour half the frosting over the cake so that the bumps are completely covered in fudge and the excess runs and pools around the sides of the pan. Tilt the pan to make sure the frosting covers the cake and bumps evenly.

Return the cake to the freezer for 5-7 mins to allow the first frosting layer to set, then remove the partially frosted cake from the freezer and cover again with the remainder of the frosting. Return the cake to the freezer for a final 15-20 mins to fully set the frosting. Allow the cake to come to room temperature before cutting and serving.