Fluffy Chocolate Cake with a Super Creamy Custard

Instructions

For the Chocolate Cake:

1. Preheat the oven to 350°F (180°C ). 
2. Line a 6-inch (15cm) round baking pan with parchment paper and set aside.
3. In a large bowl, sift together the flour, cocoa powder, brown sugar, baking powder, and salt. Mix the dry ingredients until well combined.
4. Add the egg, vegetable oil, and vanilla to the dry ingredients. Mix with a whisk until just combined. Don’t overmix.
5. Pour the batter into the prepared baking pan and bake for about 25-30 minutes or until a toothpick inserted into the center of the cake comes out clean.
6. Allow the cake to cool for a few minutes, then invert it onto a cooling rack, remove the parchment paper, and level the top. Cut the cake into three thin layers.

For the Filling: 

1. In a bowl, whisk together the egg, sugar, and cornstarch until smooth. Gradually add the heavy cream, whisking until well combined.
2. Strain the mixture into a saucepan to remove any lumps or egg membranes. 
3. Cook over medium heat, stirring constantly until the mixture begins to bubble and thicken.
4. Reduce the heat to low and continue stirring for 1 minute to cook the cornstarch completely. 
5. Turn off the heat and stir in the white chocolate and vanilla until smooth. The white chocolate helps the filling to set in the fridge.
6. Transfer the filling to a large container, cover it with plastic wrap (making sure it touches the surface to prevent skin from forming), and let it cool to room temperature. Using a larger container helps the filling to cool faster. 

Assembling the Cake:

1. Assemble the cake in the same pan used for baking. Line the pan with plastic for easy removal later. 
2. Reserve 2 tablespoons of filling for decoration. 
3. Place one layer of cake in the pan, spread half of the filling over it, add another layer of cake, the remaining filling, and finally, the last layer of cake. Close the plastic wrap and refrigerate the cake for about 2-3 hours to set.

For the Buttercream:

1. This is a quick way to make buttercream. If you prefer, you can use an electric mixer to achieve a creamier texture. 
2. Place the softened butter in a large bowl. Add the powdered sugar, cocoa powder, salt, and a tablespoon of hot water or milk. Vigorously mix the ingredients until it forms a creamy and thick frosting. Adjust the consistency by adding more powdered sugar if it's too thin or a bit more water or milk if it's too thick.
3. Invert the chilled cake onto a serving plate and remove the plastic.
4. Spread the buttercream over the top and sides of the cake, smoothing it out evenly. 

Decoration:

1. Transfer the reserved filling to a small piping bag, cut the tip, and pipe a decorative pattern on top of the cake. 
2. Decorate the center with buttercream using a piping bag fitted with a closed star tip.