Preheat oven to 350℉/176℃. Line a 9x9 baking pan with parchment paper and set aside. Zest the lemon.
1 tsp lemon zest
Add the flour, oats, baking soda, baking powder, salt, cinnamon, and cornstarch into a bowl. Stir together and set aside.
1 1/3 all-purpose flour, 1 1/4 cups rolled oats, 1 1/2 tsp cornstarch, 1/2 tsp cinnamon, 1 1/2 tsp baking powder, 1/2 tsp baking soda, 1/4 tsp salt
Into a large bowl add the butter, lemon zest, and white sugar. Cream together for 2-3 minutes on medium speed. Scrape the sides of the bowl as needed.
1/2 cup unsalted butter, 3/4 cups white sugar, 1 tsp lemon zest
Add the yogurt, vanilla, and eggs. Continue to mix for another 2 minutes, scraping the sides of the bowl as needed. The mixture may look curdled, this is normal.
2 large eggs, 1 cup yogurt, 1 tbsp vanilla
Add the dry and wet ingredients together and mix until about halfway combined with a spatula. Don't over-mix.
Into the bowl with the blueberries, add 1 tsp of flour. Dust the blueberries evenly in the flour. Pour them into the bowl with the cake batter and mix all together until no flour streaks show. Don't over-mix. As soon as there are no flour streaks, stop mixing.
2 cups fresh blueberries
Transfer into the prepared pan. Sprinkle extra blueberries on top and a bit of white sugar for a sweet top (optional).
Bake for 40-50 minutes or until a toothpick inserted comes out clean. Allow to cool, slice, and enjoy!