For topping

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8 tablespoons unsalted butter (1 stick), melted and still warm

⅓ cup granulated sugar (2 ⅔ ounces)

⅓ cup dark brown sugar (2 ⅔ ounces)

¾ teaspoon ground cinnamon

⅛ teaspoon table salt

1 ¾ cups cake flour (7 ounces)

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For cake

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1 ¼ cups cake flour (5 ounces)

½ cup granulated sugar (3 ½ ounces)

¼ teaspoon baking soda

¼ teaspoon table salt

6 tablespoons unsalted butter (¾ stick), cut into 6 pieces, softened but still cool

1 large egg

1 large egg yolk

1 teaspoon vanilla extract

⅓ cup buttermilk

Confectioners' sugar for dusting

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1. Adjust oven rack to upper-middle position and heat oven to 325 degrees. Make foil sling for 8-inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan.

2. FOR THE CRUMB TOPPING: Whisk melted butter, granulated sugar, brown sugar, cinnamon, and salt in bowl until combined. Add flour and stir with rubber spatula or wooden spoon until mixture resembles thick, cohesive dough; set aside and let cool completely, 10 to 15 minutes.

3. FOR THE CAKE: Using stand mixer fitted with paddle, mix flour, granulated sugar, baking soda, and salt on low speed until combined. Add butter, 1 piece at a time, and mix until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add buttermilk, egg and yolk, and vanilla, increase speed to medium-high, and beat until light and fluffy, about 1 minute, scraping down bowl as needed.

4. Transfer batter to prepared pan. Using rubber spatula, spread batter into even layer. Using your hands, roll pieces of crumb-topping dough between your thumb and forefinger into large pea-size pieces. Spread pieces, breaking up any large chunks, in even layer over batter, beginning with edges and working toward center. Bake until crumbs are golden and toothpick inserted in center of cake comes out clean, 35 to 40 minutes, rotating pan halfway through baking. Let cake cool in pan on wire rack for at least 30 minutes. Using foil overhang, lift cake from pan. Dust with confectioners' sugar before serving.