Rhubarb Crumb Cake

Fruit Filling (universal recipe)

This recipe works well for rhubarb, apples, cherries, peaches.
3 cups sliced rhubarb, 2 tablespoons lemon juice, 1 cup sugar, 2 tablespoons of cornstarch, 2 tablespoons butter.
In a saucepan combine the rhubarb, sugar, butter, and lemon juice. Bring to a boil and add cornstarch. Reduce heat. Simmer, uncovered until almost tender, about 7-10 minutes, being careful not to break up rhubarb. Add cornstarch and mix very well. Remove from heat. Cool to room temperature.

Crumb Topping

1/3 cup granulated sugar, 1/3 cup dark brown sugar, 3/4 tsp ground cinnamon, 1/8 tsp salt, 8 TBSP unsalted butter (1 stick), melted and still warm, 1 3/4 cups cake flour
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.

Cake

1 1/4 cups cake flour, 1/2 cup granulated sugar, 1/4 tsp baking soda, 1/4 tsp salt, 6 TBSP unsalted butter (3/4 stick), cut into 6 pieces, softened but still cool, 1 large egg, 1 large egg yolk, 1 tsp vanilla extract, 1/3 cup buttermilk
Adjust oven rack to upper-middle position and heat oven to 325 degrees. 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. Since I made individual ones, I used a muffin pan(my square one). Just butter and flour(or use baking spray).

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(or muffin pan if you are doing individual); using rubber spatula, spread batter into even layer. Evenly spoon the fruit filling on a top. Break apart crumb topping into large pea-sized pieces and spread in even layer over filling, 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(the individual ones only take about 22 minutes…watch them closely). Cool on wire rack at least 30 minutes. Remove cake from pan or from muffin tins.