Strawberry-rhubarb filling:
Make the cake:
Preheat oven to 350°F. Grease a 9-inch springform pan with a non-stick baking spray that contains butter, or butter and dust with flour.
Combine flour, baking powder, baking soda, and salt in a bowl and set aside.
With an electric mixer, beat butter and sugar until light and creamy.
Add eggs one at a time, mixing after each.
Add vanilla extract and butter milk and mix until smooth.
Add flour mixture and mix just until incorporated. Do not overmix.
Spread batter evenly into the prepared pan.
Make the filling:
Toss the strawberries, rhubarb, and lemon zest together.
Place in an even layer over the cake batter, leaving about a 1/2 inch border around the edges.
Make the crumb topping and bake cake:
Combine the sugar, flour, and melted butter with a fork until combined.
Drop in medium-sized chunks on top of the fruit filling.
Bake cake until crumb topping is golden and a toothpick inserted into the center of the cake comes out with only a few moist crumbs, about 35-40 minutes. The center of the cake should also bounce back when gently pressed.
Let cool in the pan 15 minutes. Run a knife around the edge of the cake and remove sides of springform pan. Let cool completely before serving.
Calories: 308kcal, Carbohydrates: 47g, Protein: 4g, Fat: 12g, Saturated Fat: 7g, Cholesterol: 56mg, Sodium: 169mg, Potassium: 187mg, Fiber: 1g, Sugar: 28g, Vitamin A: 401IU, Vitamin C: 16mg, Calcium: 65mg, Iron: 1mg