Preheat oven to 350F. Line the bottom of an 8-inch cake pan with parchment and spray the sides with baking spray. 

In the bowl of a stand mixer fitted with a paddle attachment, cream butter and ¾ cup sugar until light and fluffy. Add egg, vanilla, and orange zest and beat until combined. 

In a separate bowl, whisk together flour, baking soda, and salt to remove any lumps. Add half to the butter, followed by the sour cream. Add the remaining flour and the milk. Beat until combined. 

In another bowl, toss blueberries with remaining tablespoon of flour. Fold the blueberries into the batter. 

Transfer batter to the prepared pan and smooth out the top. Sprinkle with remaining tablespoon of sugar. 

Bake for 50 - 60 minutes, until a toothpick inserted into the center comes out clean. 

Let cool on wire rack before running a knife around the sides and removing from pan. Serve with whipped cream if desired.