Preheat oven to 350F (175C) and lightly grease and flour a 13×9 baking pan.⁴

Combine butter, cream cheese, and sugars and use an electric mixer (or a stand mixer) to cream together ingredients until well-combined, light, and fluffy.½ cup (113 g) unsalted butter, 8 oz (226 g) cream cheese, 1 ¼ cup (250 g) granulated sugar, ¼ cup (50 g) light brown sugar

Add eggs and vanilla extract and beat on medium-low speed until combined, pausing to scrape the sides and bottom of the bowl.2 large eggs, 1 teaspoon vanilla extract

In separate bowl whisk together flour, baking powder, cornstarch and salt.2 ½ cups (335 g) all-purpose flour, 2 teaspoons baking powder, 1 Tablespoon cornstarch, ¾ teaspoon salt

Add about ¼ of the flour mixture and stir on low speed until just combined. Add about ⅓ of the buttermilk and stir until combined. Continue to alternate adding flour and buttermilk until all are added and mixture is completely combined.½ cup (120 ml) buttermilk ¹

Use a spoon or spatula to stir in the blueberries.2 ¼ cup (330 g) blueberries²

Spread batter into prepared pan (batter will be thick) and prepare streusel, if using.Streusel

To make streusel, whisk together flour, sugars, and salt. Drizzle in melted butter and use a fork to stir/toss together until mixture is clumpy but combined (all the butter should be absorbed). Do not use butter that is too hot and do not over-mix or the mixture will be like a paste! It should be clumpy and crumbly.1 ⅓ cup (175 g) all-purpose flour, 1 cup + 2 Tablespoons (225 g) light brown sugar, 3 Tablespoons granulated sugar, ¼ teaspoons salt, 10 Tablespoons (140 g) unsalted butter

Sprinkle streusel evenly over batter.

Bake on 350F for 55-60 minutes or until a toothpick inserted in the center yields moist crumbs or no crumbs.

Allow to cool before cutting and serving.