In a large bowl, beat butter and sugar together until light and fluffy. Add the egg, egg yolk, sour cream, and vanilla, mixing until fully combined.
½ cup unsalted butter, ¾ cup sugar, 1 egg, 1 egg yolk, ½ cup sour cream, 1 teaspoon vanilla extract
In a separate bowl, sift together flour, baking powder, baking soda, and salt. Gradually fold the dry ingredients into the wet mixture until just incorporated. Avoid overmixing.
1 ⅓ cups all-purpose flour, 1 teaspoon baking powder, ¼ teaspoon baking soda, ¼ teaspoon salt
Pour the batter into the prepared pan, spreading it evenly. Sprinkle the streusel topping over the batter and lightly swirl it into the surface with a knife.
Bake for about 30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely in the pan before slicing and serving. Enjoy your coffee cake!