Ultimate Strawberry Rhubarb Coffee Cake: The Best Recipe Ever!

Make the filling: Combine sugar and cornstarch in a large saucepan. Stir in rhubarb and strawberries and bring to a simmer over medium heat; cook until thickened, about 2 minutes. Remove from the heat, stir in lemon juice, and let cool.

While the filling is cooling, preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9×13-inch baking dish.

Make the cake: Combine flour, sugar, baking powder, and baking soda in a large bowl. Cut in butter until mixture resembles coarse crumbs.

Whisk buttermilk, eggs, and vanilla together in a separate bowl. Stir wet ingredients into dry ingredients until just moistened. Spoon 2/3 of the batter into the prepared pan, spread cooled filling over top, then cover with remaining batter.

Make the topping: Combine sugar and flour in a bowl. Cut in butter until the mixture resembles coarse crumbs. Sprinkle topping over batter.

Bake in the preheated oven until topping is golden brown and a tester inserted in the center comes out clean, 45 to 50 minutes. Cool on a wire rack.