Maple tart with sour milk

Ingredients:

Cake

• 1 cup of premium flour + extra for working with the dough
• 2 tbsp fine corn flour
• 2 tablespoons powdered sugar
• 2 tbsp. ultrafine sugar
• A pinch of fine salt
• 6 tablespoons chilled unsalted butter, cut into 6 pieces
• 1 large egg yolk, lightly beaten
• Granulated sugar or decorative sugar, for sprinkling (optional)

Filling

• 2/3 tbsp. maple sugar
• 2 tbsp. premium flour
• A pinch of fine salt
• 5 tbsp butter, melt and cool slightly
• 3/4 cup sour milk or kefir
• 1 tsp vanilla extract
• 2 large eggs, lightly beaten

Preparation:

1. Cake:
   
   Place the flour, cornmeal, sugar, and salt in the bowl of a food processor fitted with the metal blade and pulse to combine. Add the pieces of butter and pulse on high to moisten the flour mixture, then continue to pulse until the mixture resembles wet sand, about 8 three-second pulses. Add the yolk and pulse again on high until the mixture is moist and just beginning to come together, about 8 more pulses. The dough should just come together when squeezed in the palm of your hand. If not, add 1 teaspoon of cold water and pulse briefly.
2. Turn the dough out onto a lightly floured work surface. Knead the dough on the work surface, flattening it with the palm of your hand until it begins to come together. Gather the dough back together and shape it into a disk. Wrap in plastic wrap and refrigerate for 1 hour.
3. Let the dough warm slightly before rolling it out, otherwise it may crack. On a lightly floured work surface, roll the dough out with a rolling pin into a circle about 0.3 cm thick and 25-27 cm in diameter. Place the dough in a 20 cm tart pan with a removable bottom. Don't worry if it cracks a little; you can patch it with scraps. Press the dough firmly into the bottom of the pan in the corners, then run the rolling pin around the edges of the pan to trim off any excess. Press the dough firmly against the sides of the pan and lift it up with your fingers (sometimes the rolling pin will push the dough down the sides a little). Refrigerate for 30 minutes.
4. Preheat oven to 350°F (175°C). Adjust the oven rack to the lowest level and place a baking sheet on it. Line the chilled dough with a 10-inch (25cm) coffee filter and weight it with dried beans or rice. Place the pan on the hot baking sheet and bake until the edges are golden brown, 15 to 20 minutes. Remove the cake from the oven, carefully remove the paper and beans, and return the pan to the oven. Bake for another 10 to 15 minutes. Reduce the heat to 325°F (160°C) and let the cake cool before adding the filling, about 10 minutes.
5. Filling:
   
   Combine the maple sugar, flour, and salt in a food processor. Add the melted butter and pulse until evenly combined, then add the buttermilk and vanilla extract and pulse until incorporated. Add the eggs and pulse until fully incorporated. Pour the filling into the tart shell. Bake the tart until the filling is set around the edges (it may rise slightly) and the center still jiggles slightly when tapped, 30 to 35 minutes. Cool the tart on a wire rack for at least 45 minutes. Serve warm or at room temperature, garnished with cookies, if desired.
6. Note
   
   You can bake cookies from the dough scraps to decorate the finished tart. Sprinkle the cookies with decorative sugar and bake them at the same time you would bake the crust for the first time; be careful not to overbake them. Alternatively, you can bake the cookies at 325°F (165°C) for 10 minutes while the finished tart cools.
   
   If the filling curls up while cooking, don't worry, you'll be fine.