Combine flour, salt, sugar, and yeast in the bowl of a mixer fitted with the dough hook (put salt on the very bottom and yeast on the very top). Add the whole eggs and the egg yolks and mix on low until the dough starts to come together, ~3 minutes. Turn up the speed to about medium and allow to knead for another 5 minutes, or until the dough starts to pull away from the bowl.

One by one, start adding cubes of butter to the bowl and continue to mix until all the butter is incorporated, 2-3 minutes.

Remove bowl from mixer and cover with plastic wrap. Set aside at warm room temperature for 1 hour. Refrigerate for 1 hour longer (this makes the dough easier to work with).

Line a baking sheet with parchment paper. Lightly flour a work surface. Divide the dough in half. One piece at a time, form the dough into a ball and flatten into a disk. Gently continue to press the dough flat and spread it into a circle that is 9-inches in diameter; start from the middle and press outward, leaving a 1-inch border, as best you can. Or make it slightly larger and build up your own edge. Take care not to stretch the dough and to keep the dough even in thickness. (You can either do the flattening on the parchment-lined tray itself, or do it on your work surface and carefully transfer it over. I did one each way - and both ways worked fine.)

Gently set a piece of plastic wrap over the dough and set aside at warm room temperature for 25 minutes.

Lemon Custard: Mix the egg yolks, sugar, lemon zest and juice together in a large mixing bowl and gradually mix in the crème fraîche. Set aside.

Preheat the oven to 180°C/360°F (or if your oven doesn't have a 360 mark, you can do what I do and just stick it somewhere between the 350 and 375 marks).

Prick the base of the dough all over with a fork. Brush the rim of both rounds with the beaten egg, and then scatter as much nib sugar over as you wish.

Divide the custard between the two rounds, filling them full. You may have just a bit of custard left over. No worries...eat that part with a spoon while you're waiting.

Carefully slide the tray into the oven and bake for 25-30 minutes, or until the edges are risen and golden brown. The filling should still look a bit wobbly, but it will be bubbling around the edges. Set the tray on a rack to cool. Cut into wedges and serve!