Prepare for Baking: Preheat your oven to 180°C (160°C fan). Grease and flour two 20cm (8-inch) round cake tins, and line the bottoms with parchment paper.

Make the Cake Batter: In a medium bowl, whisk together the plain flour, baking powder, and salt. In a separate large bowl, use a stand mixer or electric hand mixer to beat the softened butter and caster sugar on medium-high speed until light, pale, and fluffy, about 3-4 minutes.

Combine Wet Ingredients: Beat the eggs into the butter mixture one at a time, scraping down the bowl after each addition. Mix in the vanilla extract. Reduce the mixer speed to low and add the flour mixture in three additions, alternating with the milk in two additions (beginning and ending with the flour). Mix only until just combined – do not overmix.

Bake the Sponges: Divide the batter evenly between your prepared tins and smooth the tops. Bake for 30-35 minutes, or until a skewer inserted into the centre comes out clean. Let the cakes cool in their tins for 15 minutes before inverting them onto a wire rack to cool completely.

Start the Pastry Cream: While the cakes bake, make the filling. In a medium, heatproof bowl, whisk the egg yolks and sugar together until pale and thick. Whisk in the cornflour and salt until smooth.

Heat the Milk: Pour the milk into a medium saucepan and heat over a medium heat until it just begins to simmer at the edges. Don't let it boil.

Temper and Thicken: Slowly pour about half of the hot milk into the egg yolk mixture, whisking constantly to temper the eggs. Pour the tempered egg mixture back into the saucepan with the remaining milk. Cook over medium heat, whisking constantly, until the cream thickens and starts to bubble. Let it bubble for 1 minute, still whisking, then remove from the heat. I find that passing it through a fine-mesh sieve at this point guarantees a silky texture.

Finish the Pastry Cream: Whisk in the cold butter and vanilla until the butter has fully melted and the cream is smooth and glossy. Transfer the pastry cream to a bowl, press a piece of cling film directly onto the surface to prevent a skin from forming, and refrigerate for at least 2 hours, or until completely cold and set.

Make the Ganache: Place the finely chopped chocolate in a heatproof bowl. Gently heat the double cream (and golden syrup, if using) in a small saucepan until it's just simmering. Pour the hot cream over the chocolate and let it sit for 5 minutes without stirring. Then, gently whisk from the centre outwards until the ganache is smooth and glossy. Let it cool for 15-20 minutes, or until it's slightly thickened but still pourable.

Assemble Your Pie: Place one cooled cake layer on your serving plate or cake stand. Give the chilled pastry cream a good whisk to loosen it up, then spread it evenly over the cake, leaving a small border around the edge. Place the second cake layer on top. Pour the slightly cooled ganache over the top of the cake, using a spatula to gently guide it so it drips appealingly down the sides. Refrigerate for at least 30 minutes for the ganache to set before slicing and serving.