For the cake:
6 tablespoons unsalted butter, at room temperature (3/4 stick)
3/4 cup sugar
2 extra-large eggs, at room temperature
1 teaspoon pure vanilla extract
1/2 teaspoon grated lemon zest (I used 1 teaspoon to make it more lemony - hence the title!)
2/3 cup sour cream
1 1/4 cups flour
1 teaspoon baking powder
1/4 teaspoon baking soda
1/2 teaspoon kosher salt
1 cup fresh blueberries
Confectioners' sugar for sprinkling (I omitted this)
Preheat the oven to 350 degrees F. Butter and flour a 9-inch round baking pan. (I used a nine inch square glass baking dish.)
Cream the butter and sugar in the bowl of an electric mixer fitted with paddle attachment on high for 4-5 minutes, until light. Reduce speed to low and add eggs 1 at a time, then add vanilla, lemon zest and sour cream. In a separate bowl, sift together flour, baking powder, baking soda and salt. With the mixer on low speed, add flour mixture to batter until just combined. Fold in blueberries and stir with spatula to be sure batter is completely mixed.
Spoon the batter into the prepared pan and spread it out with a knife.
For the streusel topping:
4 T. melted butter
1/2 c. oats
1/2 c. flour
1/4 c. sugar
1/4 c. brown sugar
1 t. cinnamon
1/4 t. fresh ground nutmeg
Mix together all dry ingredients. Pour melted butter over the top and combine.
With your fingers, crumble the topping evenly over the batter. Bake for 40 to 50 minutes, until a cake tester comes out clean. Cool completely and serve.