For the crust:Preheat oven to 350 degrees F. Melt the butter in a saucepan or microwave.
Combine the flour, sugar, ground almonds, and salt in a medium bowl.
Pour in the melted butter and almond extract. Use a fork to combine ingredients until it forms a solid dough.Press the dough into the bottom and up the sides of a 9-inch tart pan with removable bottom. A dough tamper can help you press out the dough evenly.
Bake the tart shell for about 25-30 minutes until the shell is golden brown.
Combine the creme fraiche, egg yolk, brown sugar, and orange flower water together in a small bowl. Set aside.Slice the plums in half vertically and remove the pit. Using a sharp paring knife, cut a V-notch in the bottom of each half.
Pour the filling into the tart shell. Arrange the plum halves vertically in overlapping circles, starting from the center.
Sprinkle the sugar over the top of the plums.
Bake for 25-30 minutes, until the custard has just set and the fruit has softened and browned slightly at the edges.