Step 1: Preheat the Oven. Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan with butter or cooking spray to ensure easy removal later.
Step 2: Mix Dry Ingredients. In a large mixing bowl, whisk together 2 cups of all-purpose flour, 2 teaspoons of baking powder, and ½ teaspoon of kosher salt. Set aside.
Step 3: Cream the Butter and Sugar. In another bowl, use a hand mixer or stand mixer to cream together ½ cup of softened unsalted butter and 1 cup of sugar until light and fluffy. This should take about 3-5 minutes.
Step 4: Add Egg and Milk. Add the large egg and ½ cup of milk to the butter and sugar mixture. Beat until well combined.
Step 5: Combine Wet and Dry Ingredients. Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Be careful not to overmix, as this can make the cake dense.
Step 6: Fold in the Blueberries. Gently fold the 2 cups of blueberries into the batter using a rubber spatula, ensuring they are evenly distributed throughout the cake.
Step 7: Prepare the Topping. In a small bowl, mix together ½ cup of sugar, ½ cup of all-purpose flour, 1 teaspoon of cinnamon, and a pinch of salt. Cut in ½ cup of room temperature unsalted butter until the mixture resembles coarse crumbs.
Step 8: Assemble and Bake. Pour the blueberry batter into the prepared baking pan and spread it evenly. Sprinkle the crumb topping over the batter. Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted in the center comes out clean.
Step 9: Cool and Serve. Once baked, remove the cake from the oven and allow it to cool in the pan for about 10 minutes. Then transfer it to a cooling rack to cool completely before slicing.
