Preheat your oven to 350°F. This is a crucial first step, as it ensures that the cake bakes evenly. Grab your trusty 9×9-inch cake pan and spray it with some nonstick spray to prevent any sticking.
In a large mixing bowl, add 2 tablespoons of softened unsalted butter, 1 cup of granulated sugar, 1 large egg, and 1 teaspoon of pure vanilla extract. Using an electric mixer, beat these ingredients on medium speed. You're looking for a light, fluffy texture that indicates everything is well combined.
In another bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of fine sea salt. This mixture is vital for the cake’s structure and flavor.
With the mixer on low, gradually add half of the flour mixture to the butter mixture. Mix until just combined; it’s important not to overmix at this stage. Next, add 1 cup of buttermilk, mixing again until just combined. Finally, add the remaining flour mixture and mix until everything is incorporated.
Now, fold in 3 cups of chopped fresh or frozen rhubarb with a rubber spatula. Make sure the rhubarb is evenly distributed throughout the batter, which adds a beautiful tartness to the cake.
Spread the batter evenly into the prepared cake pan, smoothing the top with a spatula. This ensures that the cake bakes evenly and looks appealing once it's done.
For the streusel topping, in a small bowl, combine ½ cup of all-purpose flour, ½ cup of lightly packed light brown sugar, and 4 tablespoons of melted unsalted butter. Use a fork to mix until clumps form. This topping will give your cake a wonderful crunchy texture.
Sprinkle the streusel topping evenly over the batter in the pan. Don’t be shy; you want a generous layer to maximize that crunch!
Place the cake in the preheated oven and bake for 60 to 70 minutes. Keep an eye on it; when a toothpick or cake tester inserted in the center comes out clean, your cake is ready.
Once baked, allow the cake to cool in the pan for about 10 to 15 minutes before transferring it to a wire rack. Cooling is essential to let the flavors settle and enhances the cake's texture.
