INGREDIENTS
FOR THE CRUMB TOPPING:
⅓ cup granulated sugar (2 ⅔ oz)
⅓ cup dark brown sugar (2 ⅔ oz)
¾ tsp ground cinnamon
⅛ tsp table salt
8 tbsp unsalted butter (1 stick or 4 oz), melted and still warm
1 ¾ cups cake flour (7 oz) (see Note #1 below)
FOR THE CAKE:
1 ¼ cups cake flour (5 oz)
½ cup granulated sugar (3 ½ oz)
¼ tsp baking soda
¼ tsp table salt
6 tbsp unsalted butter (¾ stick or 3 oz), cut into 6 pieces, softened but still cool
1 large egg
1 large egg yolk
1 tsp pure vanilla extract
⅓ cup liquid buttermilk (not dried buttermilk powder)
Confectioners’ sugar for dusting
INSTRUCTIONS
To make the topping: Whisk sugars, cinnamon, salt, and butter in medium bowl to combine. Add flour and stir with rubber spatula or wooden spoon until mixture resembles thick, cohesive dough; set aside to cool to room temperature, 10 to 15 minutes.
To make the cake: Adjust oven rack to upper-middle position and heat oven to 325° F. Cut 16-inch length parchment paper or aluminum foil and fold lengthwise to 7-inch width. Spray 8-inch square baking dish with nonstick cooking spray and fit parchment into dish, pushing it into corners and up sides; allow excess to overhang edges of dish.
In bowl of standing mixer fitted with paddle attachment, mix flour, sugar, baking soda, and salt on low speed to combine. With mixer running at low speed, add butter one piece at a time; continue beating until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add egg, yolk, vanilla, and buttermilk; beat on medium-high speed until light and fluffy, about 1 minute, scraping once if necessary.
Transfer batter to baking pan; using rubber spatula, spread batter into even layer. Break apart crumb topping into large pea-sized pieces between your thumb, pointer, and middle fingers and spread in even layer over batter, beginning with edges and then working toward center.
Bake until crumbs are golden and wooden skewer inserted into center of cake comes out clean, 35 to 40 minutes. Cool on wire rack at least 30 minutes. Remove cake from pan by lifting parchment overhang. Dust with confectioners’ sugar just before serving.
NOTES
If you don’t have cake flour, you can make your own. You’ll need a total of 3 cups of cake flour for the crumb topping and the cake. So, make the cake flour first and then divide it up for the called-for amounts in the crumb topping and cake recipes. To do this, remove 9 tablespoons of all-purpose flour and replace it with 9 tablespoons of cornstarch. Then, sift it together 5 times before dividing it up.
It’s important to take your time when adding the crumb topping to the top of the batter. Start at the edges and work your way towards the center. This will help prevent the crumbs from sinking into the cake since the heaviest part of the batter (the center) will have some lift by the time you get there. And whatever you do, don’t pat the crumbs down – you’ll push the air out of the batter and prevent the cake from rising.
Adapted from Cook’s Illustrated, May 2007
