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1
Preheat the oven to 350F.
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2
Brush a 9 x 12 x 3-inch baking pan with butter, and set aside.
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3
Make the fruit sauce: Combine the lemon juice, cornstarch, and 1 cup sugar in a medium saucepan.
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4
Add the strawberries and rhubarb; cook, stirring frequently, over medium heat, until the rhubarb is soft and the liquid has thickened, 15 to 20 minutes.
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5
Transfer to a medium bowl; let cool.
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6
Make the crumb topping: Combine 3/4 cup sugar and 3/4 cup flour in a medium bowl.
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7
Melt 1/4 cup butter in a small saucepan over low heat.
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8
Drizzle the butter over the flour mixture; using your hands, mix until crumbly.
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9
Set aside.
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10
Make the cake batter: Whisk together the remaining 3 cups flour and 1 cup sugar, baking powder, baking soda, and salt in a large bowl.
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11
Using a pastry knife or 2 forks, cut the butter into the flour mixture until it resembles coarse meal.
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12
In a separate bowl, mix the eggs, buttermilk, and vanilla.
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13
Pour into the flour mixture; stir to combine.
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14
Spread half the cake batter evenly into the prepared pan.
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15
Top with half the fruit sauce.
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16
Carefully spread the remaining batter over the fruit, and top with the remaining fruit sauce.
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17
Sprinkle with the crumb topping.
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18
Bake until the cake is golden brown and springs back when touched in the center, about 1 hour.
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19
Transfer the pan to a wire rack to cool slightly.
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20
Serve warm or at room temperature, cut into squares.
