01 - Preheat the oven to 350°F. Grease and flour a 9-inch round or square baking pan.
02 - In a small bowl, combine sugar, flour, cinnamon, and salt. Cut in the cold butter using a fork or pastry cutter until mixture is crumbly. Refrigerate until ready to use.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt for the cake.
04 - In a large bowl, cream butter and sugar together until light and fluffy. Beat in eggs one at a time, then add lemon zest, lemon juice, and vanilla extract.
05 - Mix in half of the dry ingredients, then the sour cream, then the remaining dry ingredients until just combined.
06 - Gently fold in blueberries.
07 - Spread batter evenly into prepared pan. Sprinkle streusel topping evenly over batter.
08 - Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.
09 - Cool in pan on a wire rack for at least 20 minutes before slicing and serving.