Preheat the oven to 350°F (175°C). Grease a 9-inch cake pan with butter or non-stick spray. In a large mixing bowl, combine flour and granulated sugar. Add softened butter and use a whisk or mixer to blend until the mixture resembles coarse crumbs.
2 ½ cups all-purpose flour, 1 cup granulated sugar, 1 cup unsalted butter
In a separate bowl, whisk eggs, sour cream, and vanilla extract until smooth and combined. Pour the wet ingredients into the dry mixture, gently folding until just integrated, being careful not to overmix.
2 large eggs, 1 cup sour cream, 2 teaspoons vanilla extract
Pour half of the batter into the prepared cake pan, spreading evenly with a spatula. In a small bowl, mix powdered sugar and melted butter to create a smooth cream filling. Spread this over the first layer of batter, then pour the remaining batter on top, smoothing the surface.
1 cup powdered sugar, ½ cup unsalted butter
If desired, sprinkle chopped nuts or streusel over the top for added crunch. Place the cake in the preheated oven and bake for 35 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean.
½ cup chopped nuts or streusel (optional)
Remove the cake from the oven and let it cool in the pan for 10 minutes. Transfer to a wire rack and allow to cool completely before slicing. Serve warm or at room temperature, optionally dusted with additional powdered sugar.
