Gâteau Basque

Bring the milk to a simmer in a heavy saucepan with the vanilla pod and seeds. Meanwhile, whisk the egg yolks, caster sugar, and cornstarch together until pale and thick. When the milk steams, slowly pour it into the egg mixture while whisking constantly. Return everything to the saucepan over medium heat and whisk continuously until the cream thickens, bubbles, and holds a trail for 2 full minutes.

Pour the hot pastry cream into a flat, wide container. Press a sheet of cling film directly against the surface to prevent a skin from forming. Refrigerate until completely cold and set, at least 1 hour. Before using, beat briefly with a spatula to loosen it to a smooth, pipeable consistency.

Beat the room-temperature butter and caster sugar together in a stand mixer with the paddle attachment (or by hand) until pale, light, and noticeably increased in volume — about 4 minutes on medium speed. The mixture should look almost white and feel airy.

With the mixer running on medium, add the whole eggs one at a time, waiting until each is fully incorporated before the next. If the mixture begins to look curdled, add a spoonful of flour to stabilize it. The result should be a smooth, pale, glossy emulsion.

Sift together the flour, baking powder, and salt directly over the egg-butter mixture. Fold gently with a spatula — cut down through the center, fold up from the bottom, rotate the bowl and repeat. Stop the moment no dry flour is visible. Do not overmix — this is a short, crumbly pastry, not a developed dough.

Wrap both portions of dough in cling film, press into flat discs, and refrigerate for at least 1 hour. This resting period relaxes any gluten, firms the butter, and makes the dough workable without crumbling. Preheat the oven to 180°C (355°F) while the dough chills.

Press the larger dough portion into the base and up the sides of a buttered 22 cm tart tin. Spread the chilled pastry cream evenly over the base, leaving a 1 cm border. Roll or press the smaller dough portion into a disc to cover. Seal and crimp the edges firmly. Brush the top with beaten egg yolk and score a crosshatch pattern with a fork.

Bake at 180°C (355°F) for 35–40 minutes until deeply golden all over, with the crosshatch pattern clearly visible and the pastry sound hollow when tapped on top. Cool in the tin for 15 minutes, then unmold. Serve at room temperature — never hot, never cold.