Make the pastry: In a large bowl, rub the cold butter into the flour and icing sugar using your fingertips until the mixture resembles fine, pale breadcrumbs. You’ll feel the cool, gritty texture shift to a soft, sandy one. Stir in the egg yolk, then add the ice-cold water one tablespoon at a time, bringing the dough together with a knife until it just holds. It should feel silky, not sticky. Wrap the disc in cling film and rest it in the fridge for at least 30 minutes.

Blind bake the shell: Roll the chilled pastry out on a lightly floured surface to the thickness of a £1 coin. Gently lift it into a 23cm loose-bottomed tart tin, pressing it into the fluted edges. The pastry will feel firm and cool. Prick the base all over with a fork, line with a crumpled sheet of baking parchment, and fill with baking beans. Bake at 190°C (170°C fan) for 15 minutes, until the edges are just beginning to take on a pale golden colour. Remove the paper and beans, then bake for another 10 minutes until the base is dry and a uniform, light biscuit-brown. Set aside to cool completely on a wire rack.

Start the pudding: In a large, heavy-based saucepan, whisk together the caster sugar, cornflour, and cocoa powder. The mixture will be a fine, dark powder with no lumps. In a separate jug, whisk the whole milk, double cream, and egg yolks until smooth and pale yellow. Gradually pour the wet ingredients into the dry, whisking constantly to create a smooth, glossy slurry.

Cook the filling: Place the saucepan over a medium heat and stir continuously with a wooden spoon. At first, the mixture will be thin and milky. After about 4-5 minutes, you will feel the spoon begin to meet resistance as the mixture thickens. Listen for a soft, rhythmic bubbling sound. Once it starts to bubble, cook for exactly 2 minutes, stirring constantly. The pudding will be thick, smooth, and will coat the back of the spoon. Remove the pan from the heat.

Finish the filling: Add the finely chopped dark chocolate, vanilla extract, and a generous pinch of sea salt to the hot pudding. Stir gently until the chocolate is completely melted and the mixture is a deep, glossy, satiny brown. The aroma will be intensely rich and sweet. Pour the hot filling directly into the cooled pastry shell, smoothing the top with a spatula. The surface should look like dark, polished glass.

Chill the pie: Press a piece of cling film directly onto the surface of the hot chocolate filling to prevent a skin from forming. Place the pie in the fridge to chill for at least 3 hours, or until the filling is completely firm and set. It should feel bouncy and solid to the touch, with no wobble.

Whip the topping: Just before serving, pour the well-chilled double cream, icing sugar, and vanilla extract into a large, cold bowl. Whisk with an electric mixer on medium-high speed until the cream holds soft peaks. The cream will go from a liquid slosh to a thick, billowy cloud that holds its shape when the beaters are lifted. Spoon or pipe the cream over the set chocolate filling, spreading it to the edges. Finish with a generous scattering of chocolate shavings.