To make the dough, place the butter into a large mixing bowl and beat until fluffy. Add the sugar and gradually incorporate the eggs and egg yolks, beating constantly.

Sift together the flour, baking powder and salt. Add this to the creamed mixture, along with the zest to make a soft, sticky dough.

Cover the dough with plastic wrap and refrigerate for at least 4 to 6 hours or until it is very firm. It can be refrigerated for up to a week and is enough dough to make 3 cakes.

To make the pastry cream, in a stainless steel bowl, combine the sugar, cornstarch and egg yolks. Whisk until well blended and light in color, about 2 minutes.

In a small pot, combine the milk and vanilla and bring to a boil over medium heat. Slowly whisk the milk mixture into the egg mixture to temper. Once combined, pour the mixture back into a clean pot over low heat, whisking constantly, until it thickens and bubbles. Be careful not to scorch the bottom.

Pour the pastry cream into a shallow container and cover the surface with plastic wrap to prevent a skin from forming on the top. Refrigerate to chill.

Alternatively, you can place the pastry cream over an ice bath to cool it down very quickly. Stir continuously, as it will set rather fast. Once cool, transfer to a container, cover the surface with plastic wrap and chill.

See the Chef Notes, below, for a link to the pastry cream video recipe.

To assemble the gâteau, you will need a 9" to 10" (23-25 cm) metal baking ring.

Remove one-third of the cold dough from the refrigerator. Generously flour a large piece of parchment paper and place the dough in the center.

Roll the dough into a rough circle, until it is 1/4" thick, and a few inches larger than the diameter of the ring. Using the ring like a cookie cutter, press it into the dough. Remove the excess dough from the outside and separate it into two pieces. Store one piece in the refrigerator while you assemble the gâteau; it will be used for the top crust.

Roll out the second piece of dough into a rope long enough to line the inside of the ring. Using floured fingers, press the dough up the sides of the ring, and seal the edge and base together.

Once the wall is about 1" to 1 1/2" high (2 1/2 to 3 1/2 cm), use a pairing knife to even out the top. Peel away the excess dough and reserve it for the top crust. Gently even out the edge and carefully transfer the shell to a piece of aluminum foil. Wrap the foil up the sides of the ring and make sure it’s nice and snug.

Transfer the shell again to a baking tray, and remove the pastry cream from the refrigerator. By this time it will be cold and set, so you’ll need to whisk it with a sturdy wire whisk to smooth it out. Pour the pastry cream into the shell and smooth out the surface.

Next, roll out the reserved piece of dough, making it large enough to cover the top. The top layer will be quite thin, so be careful when transferring it to the top of the gâteau. Trim off any excess dough and press the top and edge together to seal.

Whisk a whole egg very well and gently brush the top of the gâteau. Sprinkle with a bit of sugar and place into a COLD oven. Set the oven to 325°F (163°C) and bake for approximately 1 hour, or until golden brown.

Once the gâteau has finished baking, remove from the oven and let cool for approximately 15 to 20 minutes. Run a very sharp knife between the gâteau and the ring to release. Remove the ring and let cool completely before serving. Dust with icing sugar, if desired.