Preheat oven to 375℉. Grease an 8" x 8" casserole dish and set aside.
Whisk together the flour, sugar, baking powder, and salt in a large bowl. Create a well in the center of the mixture and set aside.
Melt the 6 Tablespoons of butter in a medium bowl. Add the eggs, milk, and vanilla extract to the melted butter and whisk until combined.
Pour the wet ingredients into the center of the dry ingredients. Stir everything together until just combined.
Gently fold in the blueberries, trying not to burst them.
Transfer the cake batter to the prepared casserole dish. Smooth the top to create an even layer.
Use a teaspoon to create 12 indents on the surface of the cake. Place a teaspoon of cream cheese into each indent.
Mix the cold butter, additional flour, and cinnamon in a small bowl until the mixture resembles coarse sand.
Evenly sprinkle the streusel topping over the top of the cake batter.
Bake in the preheated oven for 22-28 minutes, or until the cake is lightly golden brown and a toothpick inserted into the center comes out clean.
Notes
Avoid Overmixing the Batter: To ensure your coffee cake has a tender crumb, mix the batter until just combined. Overmixing can lead to a dense cake.
Room Temperature Ingredients: For the best results, have your eggs, milk, and cream cheese at room temperature before starting. This ensures a smoother batter and a more evenly baked cake.
Checking for Doneness: Ovens can vary, so start checking your cake a few minutes before the recommended baking time. The cake is done when a toothpick inserted into the center comes out clean.
Adding Cream Cheese: When adding the cream cheese to the batter, try to space the dollops evenly to ensure each slice has a creamy pocket of cheese.
Preventing the Topping from Burning: If you notice the streusel topping getting too brown before the cake is set, lightly cover the top with aluminum foil and continue baking.