
Our strawberry rhubarb coffee cake is a slice of spring. Serve it at room temperature with a cup of coffee or warm with ice cream and a glass of dessert wine.
In a large saucepan, combine the rhubarb, strawberries and lemon juice. Cover the saucepan with the lid and cook the mixture over medium heat for five minutes. In a bowl, whisk together the sugar and cornstarch, then stir the mixture into the saucepan. Bring the mixture to a boil, and cook and stir until it’s thickened, about two minutes. Remove the saucepan from the heat and set it aside.
Preheat the oven to 350°F. In a large bowl, whisk together the all-purpose flour, sugar, baking powder, baking soda and salt. Cut in the butter until the mixture resembles coarse crumbs. In a separate bowl, use a hand mixer or stand mixer to beat the buttermilk, eggs and vanilla. Stir the buttermilk mixture into the crumb mixture just until combined.
Spread half the cake batter evenly into a greased 13x9-inch baking dish. Carefully spread the strawberry rhubarb filling on top, making sure to reach all four corners. Using a spoon or cookie scoop, drop the remaining batter by the tablespoonfuls over the filling.
Melt the butter in a saucepan over low heat. Remove the saucepan from the heat, then stir in the all-purpose flour and sugar until the mixture resembles coarse crumbs. Sprinkle the streusel evenly over the batter. Lay a large piece of aluminum foil on the lower oven rack to catch any juice spillovers.
Place the rhubarb strawberry coffee cake on the middle rack of the preheated oven. Bake the coffee cake for 40 to 45 minutes. Transfer the coffee cake to a wire rack and let it cool before enjoying.