250 grams (2 cups) all-purpose flour
250 grams (2 1/2 cups) hazelnut flour
250 grams (1 1/4 cups) granulated sugar
6 grams (1 1/2 teaspoons) fleur de sel
250 grams (2 1/4 sticks) unsalted butter, at room temperature
Preheat the oven: Preheat the oven to 350°F (175°C). Line a sheet pan with parchment paper.
Make the streusel dough: In a stand mixer fitted with the paddle attachment (or in a large bowl using a spatula), combine the all-purpose flour, hazelnut flour, sugar, and fleur de sel. Add the butter and mix to combine. Using your fingers, break up the streusel dough into pieces about the size of marbles. (Alternatively, push the dough through the holes of a wire rack to get pieces of a more uniform size.)
Bake the streusel: Spread the streusel over the prepared pan and bake until golden, 10 to 12 minutes. The edges should be crunchy and the inside still soft. Let cool completely.
If you can’t find hazelnut flour, pulse 250 grams (3 cups) whole hazelnuts in a food processor or blender until finely ground. You can also substitute almond, pistachio, or walnut flour.