Breton Buckwheat Butter Cake

Ingredients

• ¾ cup (102 g) all-purpose flour
• ¼ cup (30 g) buckwheat flour
• 1¼ teaspoons baking powder
• ¼ teaspoon freshly grated nutmeg
• 1 large egg, at room temperature
• 2 large egg yolks, at room temperature
• 10 tablespoons (5 oz/140 g) unsalted butter, at room temperature
• ⅓ cup (67 g) granulated sugar
• ⅓ cup (67 g) packed light brown sugar
• ½ teaspoon fleur de sel, or ¼ teaspoon fine sea salt, plus more to taste
• 1 large egg yolk, for the optional glaze
• Toasted buckwheat groats (kasha), for topping (optional)
• Fleur de sel, for topping (optional)
• Turbinado sugar, for topping (optional)

Instructions

1. Center a rack in the oven and preheat to 350°F (175°C). Generously butter a 9- to 9½-inch fluted tart pan with a removable bottom. If using a springform or regular round cake pan, butter it and line the bottom with parchment.
2. In a medium bowl, whisk together the all-purpose flour, buckwheat flour, baking powder, and nutmeg.
3. In a large bowl with a hand mixer, or in a stand mixer fitted with the paddle, beat the softened butter, granulated sugar, brown sugar, and fleur de sel on medium speed until smooth and creamy, about 2 minutes. The mixture does not need to become very fluffy.
4. Beat in the whole egg until fully incorporated. Add the 2 egg yolks one at a time, mixing well and scraping the bowl after each addition.
5. Add the dry ingredients and mix on low only until the flour disappears. Finish folding with a spatula. The dough will be soft, thick, and sticky, more like shortbread dough than cake batter.
6. Transfer the dough to the prepared pan. With a lightly buttered spatula or damp fingertips, press it into a thin, even layer and make sure it reaches the fluted edges.
7. For the optional glossy topping, beat the remaining egg yolk until smooth and brush a thin coat over the surface. Sprinkle lightly with toasted buckwheat groats, turbinado sugar, and a small pinch of fleur de sel.
8. Bake for 25 to 30 minutes, until the cake is deeply golden, firm around the edges, and just set in the center. A tester may carry a few moist crumbs but should not show raw dough.
9. Cool in the pan on a rack for 10 minutes, then carefully remove the outer ring. Let the cake cool completely before sliding it from the pan base and cutting it into 8 wedges.