Rhubarb Galettes With Hazelnut Frangipane

Pulse flour, granulated sugar, and salt in a food processor to combine. Add butter and process until mixture resembles coarse meal with a few pea-size pieces of butter remaining.
Transfer mixture to a large bowl; drizzle 1/3 cup ice water over. Gently mix with your hands until a shaggy dough forms.
Turn out dough onto a lightly floured surface and knead once or twice, pressing to incorporate any dry bits of flour. Divide dough into 2 pieces and flatten each into about a 1"-thick disk. Wrap in plastic wrap; chill at least 1 hour.
Dough can be made 2 days ahead. Keep chilled, or freeze 1 month.
Preheat oven to 350°F. Process hazelnuts and 1/3 cup granulated sugar in a food processor to a fine paste, about 3 minutes. Add butter, buckwheat flour, rye, salt, and 1 egg and pulse until smooth; set frangipane aside.
Toss rhubarb, orange zest and juice, and remaining 1/4 cup granulated sugar in a medium bowl. Let sit, tossing occasionally, 10 minutes.
Whisk remaining egg in a bowl with 1 teaspoon water. Divide each disk of dough into 4 pieces. Roll out a piece of dough on a lightly floured surface to make a 5" round. Spread with about 1 1/2 tablespoons reserved frangipane, leaving a 1" border. Brush border with egg wash. Top frangipane with some rhubarb; fold edges of dough up and over rhubarb, overlapping slightly. Brush dough with egg wash. Repeat process to make 7 more galettes. Transfer to parchment-lined baking sheets.
Sprinkle galettes with demerara sugar and aniseed and bake, rotating once, until crust is golden brown and rhubarb is soft, 30–40 minutes. Let cool on a wire rack. Do ahead: Galettes can be baked 3 days ahead; store tightly wrapped at room temperature.