CHOCOLATE BROWNIE SHEET CAKE

1. Preheat the oven to 350° / 180°. Grease and line a 9”x13” baking pan with parchment paper.
2. In a small saucepan over medium heat, heat the butter and chocolate, stirring constantly, until melted and combined. Set aside to cool.
3. In a small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
4. In a large bowl, whisk together the sugar, dark brown sugar, eggs, vanilla, and oil. Add the cooled chocolate mixture, and mix to combine. Add the dry ingredients, and gently fold to combine.
5. Transfer the batter to the prepared baking pan and bang on the counter once or twice to help it settle.
6. Bake for 45 to 50 minutes, checking for doneness after 45 minutes, until a the top of the cake is set, and skewer inserted in the middle of the cake comes out with a few crumbs attached.
7. Allow to cool completely in the baking pan.

CHOCOLATE HAZELNUT GERMAN BUTTERCREAM

1. In a bowl, whisk together the sugar and corn starch, then add the egg, egg yolk and salt, and whisk together well. In a medium non-stick saucepan, heat the milk and vanilla bean paste to just shy of a simmer. Remove from the heat.
2. Using one hand to whisk constantly, pour half of the hot milk mixture into the egg mixture. This helps to temper the eggs and stop them from scrambling. Whisk until incorporated, and then pour the whole lot back into the saucepan.
3. Heat the milk and egg mixture over medium heat, whisking constantly, until it begins to bubble. It will thicken quickly. Once it has thickened, cook for one minute, then remove from the heat. Pour into a shallow dish or bowl of a stand mixer and press some plastic wrap over the surface to avoid a skin from forming. Refrigerate until cold.
4. If you are in a hurry, you can speed up this process by placing the custard mixture into a bowl, and placing the bowl into an ice bath, stirring frequently.
5. Fit your mixer with the whisk attachment. Whip the custard mixture on medium until creamy and lump-free. Begin adding the butter, a few cubes at a time, until fully incorporated.
6. Add in the chocolate hazelnut spread, and whip until smooth, then whip for a further 2-3 minutes until light and fluffy. Switch to the paddle attachment and beat on low for 3-4 minutes to help remove any big air bubbles.

ASSEMBLY

1. Carefully remove the cooled cake from the pan - I do this by turning it onto a wire rack, then using another wire rack with parchment paper on top to flip it up the right way - alternatively you can also put the buttercream on while it is still in the pan, which makes it super easy to transport.
2. Scrape the buttercream onto the cake, and spread using an offset spatula - swoop it on as desired. Finish with sprinkles if using.
3. Store leftovers in an airtight container at room temperature or the fridge.