In a large mixing bowl, whisk 3 eggs and 200 g sugar until pale and slightly thickened, about 2 minutes.

Whisk in 200 ml vegetable oil and 1 tsp vanilla extract until smooth.

Sift in 500 g flour, 1 tbsp baking powder, and 1/4 tsp salt. Mix with a wooden spoon, then knead briefly by hand into a soft dough. Divide into 5 equal pieces, about 180 g each, and rest for 15 minutes.

Heat the oven to 180 C / 350 F and line two sheet pans, 33 x 46 cm / 13 x 18-inch, with parchment.

Roll one piece of dough directly on parchment into a thin round about 24 cm / 9.5-inch across, using the base of the springform pan as a guide to trim the edge. Prick all over with a fork.

Bake one layer at a time for 7 to 9 minutes, until the edges turn light golden. Watch closely after 7 minutes, the thin dough can go from golden to burnt fast. Cool each layer completely on a rack before stacking.

In a 3-quart / 2.8 L saucepan, whisk the milk, 150 g sugar, 60 g cocoa powder, 60 g cornstarch, 2 egg yolks, and a pinch of salt until smooth with no lumps.

Set over medium heat and cook, whisking constantly, for 6 to 8 minutes, until the mixture thickens enough to coat the back of a spoon.

Remove from heat and stir in 50 g butter and 1 tsp vanilla extract until glossy. Press plastic wrap directly onto the surface and cool to room temperature, about 30 minutes.

Place one dough layer on a serving plate. Spread about 120 g of the cooled filling evenly to the edges. Repeat with the remaining layers, finishing with a plain layer on top.

Spread any remaining filling thinly over the top layer, then dust with 2 tbsp cocoa powder through a fine sieve.

Cover loosely and chill for at least 4 hours, ideally overnight, until the layers soften enough to slice cleanly. Cut with a knife dipped in hot water and wiped dry between slices.