Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 9-inch square baking pan or a 9-inch round springform pan.
Step 2: Prepare the Rhubarb Filling: In a medium bowl, combine the diced rhubarb with 1/4 cup granulated sugar and 1 tablespoon all-purpose flour. Toss gently until evenly coated. Set aside.
Step 3: Make the Streusel Topping: In a separate small bowl, whisk together 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, and 1/2 teaspoon ground cinnamon. Pour in the 1/4 cup melted unsalted butter and mix with a fork until moist crumbs form. Set aside.
Step 4: Prepare the Cake Batter (Dry Ingredients): In a medium bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.
Step 5: Prepare the Cake Batter (Wet Ingredients): In a large bowl, using an electric mixer, cream together 1/2 cup softened unsalted butter and 1 cup granulated sugar until light and fluffy, about 2-3 minutes. Beat in the large egg until well combined, then stir in 1 teaspoon vanilla extract.
Step 6: Combine Cake Batter: Gradually add the dry ingredients to the wet ingredients, alternating with the 1 cup buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix.
Step 7: Assemble Cake: Pour about two-thirds of the cake batter into your prepared baking pan, spreading it evenly. Spoon the rhubarb filling over the batter. Gently dollop the remaining cake batter over the rhubarb.
Step 8: Top and Bake: Evenly sprinkle the prepared streusel topping over the entire cake. Place the pan in the preheated oven and bake for 45-50 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean and the streusel is golden brown.
Step 9: Cool and Serve: Let the coffee cake cool in the pan on a wire rack for at least 15-20 minutes before serving. This allows it to set properly.