Preheat and prepare pan. Preheat oven to 350°F (180°C). Grease a 8 or 9-inch square pan. You can line the pan with parchment paper if you want to remove the cake after baking. For a thicker cake use an 8-inch pan. Set aside.
Combine dry ingredients. In a mixing bowl, whisk together flour, baking powder, and salt. Set aside. 1 ½ cups + 2 tablespoons (195 g) all purpose flour 1 ½ teaspoons baking powder ½ teaspoon salt
Cream butter oil and sugar. In a second mixing bowl with an electric mixer on medium speed, cream butter, oil, sugar, and lemon zest together until smooth and combined for about 2 to 3 minutes. ¼ cup (½ stick, 57 g) unsalted butter ¼ cup (60 mL) neutral-tasting oil (I used canola oil) 1 cup (200 g) granulated white sugar Zest of 1 lemon
Add eggs, vanilla. Add the eggs one at a time, beating well after each addition. Scrape down the sides and bottom of the bowl after each addition. Add vanilla extract. Beat to combine. 2 large eggs 2 teaspoons vanilla extract
Alternate dry ingredients with buttermilk. Alternate adding the flour mixture with the buttermilk, beginning and ending with the flour mixture. Do not overmix. Stop mixing once the last of the flour has been mixed in. Fold in 1 ½ cups of the blueberries (leave any excess flour behind in the bowl). ¾ cups (180 mL) buttermilk
Bake. Spoon cake batter into the prepared cake pan, smooth the top. Sprinkle on the remaining ½ cup of blueberries. Sprinkle on about a tablespoon of granulated sugar. Bake for about 40-45 minutes (it will take slightly longer in an 8-inch cake pan, and if you use frozen berries). Start checking at 35 minutes. The cake is done when it's golden brown on top and toothpick inserted into the center comes out clean or with a few moist crumbs. Remove cake from the oven. Cool completely then slice and serve.