Pre-heat oven to 350 degrees. Butter and flour a 9-inch round cake pan. Set aside.
Make the cake first. Cream butter and sugar in the bowl of an electric mixer fitted with the paddle attachment until light and fluffy. Reduce speed to low and add eggs, one at a time.
Then, add vanilla, lemon zest and sour cream.
While that is mixing, prepare your dry ingredients. Sift the flower, baking powder, baking soda and salt in a medium bowl. Working in batches, slowing add dry ingredients to your wet mixture until all has been incorporated. You may have to scrape down sides with a spatula.
Fold in blueberries and give a stir with your spatula to ensure all batter is mixed.
Pour mixture into your floured and buttered cake pan.
Prepare the streusel. Combine both sugars, cinnamon and nutmeg. Stir in melted butter and then flour. With your hands, mix until a pasty dough is formed.
Crumble evenly over cake.
Bake for 40 minutes or until a cake tester comes out clean.
Cool completely and serve sprinkled with confectioners’ sugar.