For the Cake Batter

• Preheat the oven to 350°F with the rack placed in the middle.

  Grease and flour a 9x13x2" baking pan; or use a piece of parchment paper to line it.
• In a medium bowl, whisk the flour, baking powder, and salt.
• In the bowl of a stand mixer or a large bowl for mixing with a hand mixer, cream together the butter, granulated white sugar and light brown sugar on medium-high speed until fluffly, about 2 minutes. Scrape down the sides and bottom of the bowl halfway through with a spatula.
• While the mixer is running, add the eggs one at a time until each one is incorporated. Decrease the speed and add half the flour until incorporated, then add the next half. Mix and increase the speed to medium for a minute.
• In a separate small bowl, whisk the milk, sour cream and vanilla extract. Add this into the mixer bowl over medium speed and mix until combined. Use a spatula to scrape the sides and bottom of the bowl and mix to finish.
• Toss half of the frozen blueberries with 1 teaspoon flour and carefully fold into the batter. Use a light hand and few strokes to prevent the berries from bleeding too much and turning your cake green.
• Pour batter into prepared pan and spread the top evenly. Scatter the remaining frozen blueberries over the cake batter.
• For the Topping: Stir the cinnamon and sugar in a small bowl and sprinkle over the batter with blueberries on top.
• Place the pan in the preheated oven and bake for 40-45 minutes, rotating the pan 180° halfway through. The cake is done when a toothpick or cake tester inserted in the center comes out clean. The top will be golden brown. Cool the cake in the pan for 20 minutes before serving.