Preheat the oven to 350°F and position a rack in the center. In a bowl, combine the flour, baking powder, salt, and granulated sugar with a whisk. Slice the butter into tiny bits and then add to the mixture. Work in the butter with your fingers, until only pea-sized lumps remain. Add just the egg yolk. Mash it into your flour and butter and knead it with your hands until it becomes a dough. Form the dough into an oval disc, wrap in saran wrap and place in the fridge to chill for 30 minutes.
Remove the dough from the fridge and roll it out until it's around ¼ inch thick. Grease your cake pan or tart pan and carefully place the dough into your pan. Press the dough firmly into the pan and cut off any access that hangs over. You can also pinch the dough around the border like you would a pie (if you are using a cake pan).
Place a piece of aluminum foil in the pan and pour on pie weights or a few cups of rice to keep the foil down in the crust. Bake the tart crust for 30 minutes. Remove from the oven and remove the aluminum foil and rice (or pie weights). With a pastry brush, coat the crust with the remaining egg white. Bake in the oven once more (without foil) and bake for an additional 8-10 minutes until the crust becomes a golden but not super baked brown color. Remove from the oven and let cool.
To make the filling, start by heating the cream on medium heat in a small pot until the cream begins to simmer and slightly bubble. Remove from the heat and add in the chocolate and the vanilla and stir until melted. Add the sugar and salt and stir together until the mixture is smooth. In a small, separate bowl, beat the two eggs with a fork before pouring into the cream. Stir until well blended.
Adjust your oven temperature to 325°F. Place the tart or cake pan on top of a baking sheet for easy transfer. Carefully pour the filling into the tart pan. Do not let it overflow.
Move the pan to the oven and bake for 15 minutes or until the surface looks firm but glossy. It should NOT be cracking as that is a sign of over baking. Let cool.
Slice and enjoy as it or top with sliced peaches and thyme.