2/3 cup all-purpose (plain) flour, plus more for dusting
1 1/2 tablespoons buckwheat flour
2 tablespoons cornstarch
Pinch of kosher salt
1/3 cup unsalted butter
1/4 cup sugar
1/4 teaspoon vanilla extract
For the praline feuilletine:
2/3 cup hazelnuts
1/2 cup sugar
1/2 cup water
1 tablespoon glucose
3 1/3 tablespoons feuilletine
1 1/4 teaspoons kosher salt
For the assembly:
Chocolate-hazelnut crémeux (recipe below)
Flaky sea salt
For the pâte sucrée: In a medium bowl, whisk together the all-purpose flour, buckwheat flour, cornstarch and salt.
In a bowl, with an electric mixer on medium-high speed, cream the butter, sugar and vanilla. Add the egg, scrape down the sides of the bowl, and continue to beat until well combined. With the mixer on low, add the flour mixture and beat until well combined.
Wrap the dough in plastic wrap and pat to a 1-inch-thick square. Refrigerate for at least 20 minutes.
Preheat the oven to 325 degrees. Line a baking sheet with a silicone baking mat.
Roll the dough on a lightly floured surface to a thickness of about 1/16 inch. Using a 5-inch ring cutter, punch out rounds (you can chill it if it’s feeling too soft to cut).
Shimmy four rounds of dough into four 4-inch ring molds, pressing to get into the edges of the ring mold. Place on the lined baking sheet.
Bake until the dough is evenly golden brown, 12 to 15 minutes. (Feel free to do this with the remaining dough and eat them as snacks.)
For the praline feuilletine: Preheat the oven to 325 degrees.
Spread the hazelnuts on a sheet pan and bake until golden brown and fragrant, 10 to 15 minutes.
Line a second sheet pan with a silicone baking mat. In a large frying pan over medium heat, combine the sugar, glucose, and just enough water to make the texture of wet sand. Once the caramel is a light-amber honey color, add the hazelnuts and stir to coat. Remove from the heat and quickly transfer the hazelnuts to the lined sheet pan and let cool completely.
Break up the praline and transfer to a food processor. Pulverize until the texture is slightly grainy and chunky but won’t break down further. Add a bit of canola oil and continue to process until the praline is smooth and the consistency of loose peanut butter. Transfer to a large bowl with the feuilletine and salt and mix to coat the feuilletine evenly. Set aside.
For the assembly: Spoon a bit of the praline feuilletine into the bottom of the tart shell, using the bottom of a spoon to make an even layer. Pipe a generous amount of chocolate-hazelnut crémeux into the shell. Finish with flaky sea salt.