Prepare the rhubarb: Toss the rhubarb pieces with 50g of the caster sugar and the lemon zest in a bowl. Let it sit for 15 minutes — you’ll notice it release a pinkish syrup, which will intensify the colour and flavour during baking.

Preheat your oven: Set it to 180°C (160°C fan) and line a 23cm round springform tin with parchment paper. The sides should be well greased so the crispy top doesn’t stick.

Cream the butter and sugar: In a stand mixer or large bowl, beat the softened butter with the remaining 150g caster sugar until pale and fluffy, about 3-4 minutes. You’ll know it’s ready when the mixture looks light and feels airy against the whisk.

Add the eggs and vanilla: Beat in the eggs one at a time, followed by the vanilla extract. The batter should look smooth and glossy, with no streaks of egg visible. If it looks like it’s curdling, add a tablespoon of the flour to bring it back together.

Combine the dry ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Fold half of this into the butter mixture gently, then stir in half the milk. Repeat with the remaining flour and milk until just combined — the batter will be thick but spoonable, with a soft, silky sheen.

Layer the batter and rhubarb: Spoon half the batter into the prepared tin and smooth the top. Scatter the macerated rhubarb evenly over the batter (including any syrup), then dollop the remaining batter on top. Use an offset spatula to gently spread it — don’t worry if some rhubarb peeks through, it adds character.

Add the crispy topping: Sprinkle the demerara sugar evenly over the top of the cake. This is the secret to that crackling crust — the coarse sugar melts and caramelises into a brittle, golden shell. Press it lightly into the batter with your fingertips.

Bake until golden and crisp: Bake for 45-50 minutes. At the 40-minute mark, check the top — it should be deep amber and the edges should be pulling away from the tin. Insert a skewer into the centre; it should come out clean or with a few moist crumbs. The kitchen will smell like toasted sugar and tart fruit.

Cool in the tin: Let the cake rest in the tin on a wire rack for 15 minutes. This allows the structure to set, so the crisp top doesn’t shatter into pieces when you remove the sides. Then release the springform and let it cool completely.

Dust and serve: Just before serving, dust with a fine layer of icing sugar through a sieve. The white powder against the amber crust makes it look utterly professional. Slice with a serrated knife for clean cuts.