Make the Pastry: In a large bowl, mix the 225g plain flour, 1/4 tsp salt, and 1 tbsp caster sugar. Add the 140g of cold, cubed butter. Using your fingertips, rub the butter into the flour until the mixture resembles coarse breadcrumbs. Mix the egg yolk with 2 tbsp of ice-cold water and add it to the flour mixture. Bring it together with your hands or a knife until it just forms a dough. If it's too dry, add the final tbsp of water. Do not overwork it.

Chill the Dough: Form the dough into a flat disc, wrap it tightly in cling film, and chill in the fridge for at least 30 minutes.

Prepare the Pie Dish: Preheat your oven to 200°C (180°C fan). On a lightly floured surface, roll out the chilled dough to about the thickness of a £1 coin. Carefully line a 23cm loose-bottomed tart tin or pie dish with the pastry, trimming any excess. Prick the base with a fork.

Blind Bake the Crust: Line the pastry case with baking parchment and fill with baking beans or uncooked rice. Bake for 15 minutes. Carefully remove the paper and beans and bake for another 5 minutes until the pastry is pale golden. Reduce the oven temperature to 180°C (160°C fan).

Start the Brownie Filling: While the crust bakes, place the 150g butter and 150g chopped dark chocolate in a heatproof bowl set over a saucepan of gently simmering water (a bain-marie). Stir occasionally until melted and smooth. Remove from the heat and set aside to cool slightly for 5-10 minutes.

Mix Wet Ingredients: In a separate large bowl, using an electric mixer or a whisk, beat the 2 large eggs, 100g caster sugar, and 100g light brown sugar for 3-4 minutes until pale, thick, and voluminous. This step is key for the crackly top. Stir in the 1 tsp vanilla extract.

Combine and Finish the Batter: Pour the slightly cooled chocolate-butter mixture into the egg mixture and gently fold to combine. In another bowl, sift the 75g plain flour, 25g cocoa powder, 1/2 tsp salt, and 1 tsp espresso powder. Add the dry ingredients to the wet ingredients and fold until just combined. I find that it's important not to overmix here; stop as soon as you no longer see streaks of flour.

Bake the Pie: Pour the brownie batter into the partially baked pie crust and spread it evenly. Bake for 30-35 minutes. The edges should be set and slightly puffed, but the centre should still have a definite wobble.

Cool and Serve: Let the brownie pie cool in the tin on a wire rack for at least 30 minutes before slicing. This allows the centre to set to a perfectly fudgy consistency. Serve warm, perhaps with a dusting of icing sugar or a dollop of cream.