Crust and crumbs
3/4 cup unsalted butter (1 1/2 sticks), chilled, cut into small pieces, plus more for pan
1/3 cup (1 3/4 ounces of 49 grams) hazelnuts
1 1/2 cups (188 grams) all-purpose flour
1/2 cup granulated sugar
1/3 cup packed light or dark brown sugar
1/2 teaspoon ground cinnamon
1/2 teaspoon table salt

Filling
1 pound ripe but firm plums (about 4 standard black ones or 12 smaller Italian plums; I used a mix of both)
1 tablespoon (8 grams) all-purpose flour
1/4 cup plus 2 tablespoons (6 tablespoons) granulated sugar
1 large egg, lightly beaten
1 large egg yolk
1/3 cup heavy cream
1/4 cup milk
1/4 teaspoon table salt
1/4 teaspoon ground cinnamon
Grated nutmeg, optional