
Peel the lemon in strips using a vegetable peeler and place it in a large saucepan with the milk and vanilla extract. Heat the milk until it just starts to simmer then turn off the heat immediately (photo 1).
Meanwhile, in a large bowl, combine the egg yolks, sugar and cornstarch and whisk until smooth and combined (photo 2).
Add a splash of the hot milk to the egg mixture and whisk it in. Place a sieve over the bowl and pour in the remaining milk, straining out the lemon peel. Give it a good whisk to avoid the eggs overcooking (photos 3 and 4).
Pour the mixture back into the large saucepan you heated the milk in and heat on a medium-low heat until very thick. I use a wooden spoon until I can feel the pastry cream starting to thicken, then switch to a hand whisk to avoid any lumps.
Pour the pastry cream into a clean bowl and cover it with plastic wrap, directly on top of the cream to stop a skin from forming. Let it cool for 20-30 minutes (photos 5 and 6).
Preheat the oven to 180C (350F) static (non-fan oven). Grease your cake pan with butter and line the bottom with parchment paper. Place the cake pan on a baking tray and set aside (photo 7).
Melt the butter in a saucepan and let it cool (warm is fine, just not hot). Warm the milk in a separate saucepan so it’s not fridge cold; it should be tepid and not hot.
Add the flour and baking powder to a bowl and give it a quick whisk to aerate it (photo 8).
Put the eggs, sugar and lemon zest to a the bowl of a stand mixer (or use a large mixing bowl and an electric hand whisk). Beat with the whisk attachment on medium speed. Once the mixture combines, continue to beat it for 2 minutes; it should be frothy (photos 9 and 10).
Next, add the melted butter and whisk it in, followed by the milk.
While whisking, add the flour and baking powder a spoonful at a time until it is all used up. Whisk for a few seconds until thoroughly combined (make sure to scrape down the sides of any excess flour and whisk it in) (photo 11).
Pour the batter into your prepared cake pan, then dollop over spoonfuls of pastry cream. I use roughly 2 tablespoons of pastry cream per dollop and place them in a circle around the outside, then in the middle (photos 12 and 13).
Bake in the oven for 35 minutes (check with a skewer inserted into the sponge). Remove from the oven and let it sit for 10 minutes before removing the springform ring (if using) (photo 14).
Once completely cool and just before serving, dust some powdered sugar over the top, cut into slices and serve.