Make the custard: pour the Whole Milk into a saucepan with the zest of the Lemon and a splash of Vanilla Extract. Warm gently until just below boiling, then remove from the heat and let the lemon zest infuse for 5 minutes.
Whisk together the Sugar, Egg Yolks and Cornstarch in a bowl until smooth and pale. Temper the yolk mixture by slowly whisking in about a third of the hot milk, then return everything to the saucepan.
Cook the mixture over medium heat, stirring constantly, until it thickens to a velvety pastry cream. Remove from heat, discard the lemon zest, and stir in the Unsalted Butter until glossy. Press plastic wrap directly onto the surface and chill until cool and spreadable.
Preheat the oven to 200°C (400°F). Roll the Puff Pastry out to a roughly even rectangle about 1 cm thick and cut into two equal rectangles. Transfer the bottom rectangle to a parchment-lined baking sheet and prick it lightly all over with a fork.
Beat one Eggs to make an egg wash. Brush the edges of the bottom pastry with some of the egg wash, then bake the bottom layer for about 10–12 minutes until set and starting to color (you can tent the center with foil if it rises too much).
Let the baked base cool slightly. Spread or pipe the chilled custard over the base, leaving a small border. Top with the second pastry rectangle, pressing the edges to seal the filling inside.
Brush the top with the remaining egg wash and scatter the Sliced Almonds generously over the surface. Return to the oven and bake 18–25 minutes more, until the top is deep golden and the almonds are toasted.
Cool the pantxineta completely so the custard firms up, then finish by dusting with Powdered Sugar. Slice into squares and serve at room temperature.
