In a food processor bowl add the flour, powdered sugar, and a pinch of salt. Pulse 1 – 2 times to mix. Add the butter cubes and pulse until dough is crumbly with peas-sized pieces. Add about 2 tablespoons cold water and pulse into a smooth dough. Wrap in foil and refrigerate for at least 30 minutes.
Preheat oven to 375°F. Brush a tartlet mold with butter. I just used a 12-cup muffin tin.
Roll butter pastry on a floured work surface. Then cut with a cookie cutter circles of about 3-inches in diameter. Carefully place them in the molds and press all the way to the edge. Cut away the overhanging dough with a knife.
Place some dough weights or beans in cups to weigh the dough down to prevent shrinking or buckling. Bake for 10 minutes, remove the weights and bake another 5 – 8 minutes, or until golden brown. .
In the meantime, zest and juice the lemons.
Separate eggs. Add the egg yolks to the lemon juice, zest, 1/2 cup sugar, cornstarch and 1/2 cup water in a saucepan, stir well and heat on the stove with constant stirring until a pudding-like consistency. Stir in crème fraîche.
Carefully pour the lemon cream into the pastry tarts and let cool in the fridge.
For the meringue beat the egg whites with a pinch of salt until stiff, gradually adding 1/4 cup sugar.
Pour egg whites into a piping bag with star spout and pipe on the lemon cream. Flush the meringue briefly with a flambéer.