• Make the topping:
• In a medium bowl, stir 1 cup flour, ¼ cup sugar, ⅓ cup brown sugar, 1 teaspoon ground cinnamon, and 1 dash salt until well blended. Add 7 Tablespoons melted butter and mix with a fork, stirring until the butter is absorbed and the dry ingredients are uniformly moistened. Set aside.
• Make The Cake:
• Preheat oven to 350°F, placing rack in center. Butter the bottom and sides of a 9-inch springform pan and line the bottom with parchment paper. You may use a regular 9-inch round or square pan if you don't have a springform pan.
• In a medium bowl, whisk together 1 ½ cups flour, 1 ¼ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon lemon zest, and ½ teaspoon salt.
• In a small bowl, toss 1 cup blueberries with 1 Tablespoon of the flour mixture until the berries are coated; set aside. In another small bowl, whisk together 1 cup sour cream and 2 teaspoons vanilla extract.
• In the bowl of a stand mixer, or with an electric mixer, beat together 10 Tablespoons butter and 1 cup granulated sugar a medium speed until light and fluffy, for about 4 minutes, scraping down the sides of the bowl halfway through. Beat in 2 eggs one at a time, scraping down the sides of the bowl as necessary. On low speed, add the flour mixture in three additions, alternating it with the sour cream in two additions. Using a rubber spatula, fold in the blueberries (If you mix with the mixer, some of the berries break down, which is still okay). The cake batter will be very thick; scrape it into the prepared pan and smooth the top with a spatula.
• Sprinkle all of the crumb topping evenly over the batter. Break down really large clumps, but you want some to remain.
• Bake the cake until a toothpick inserted into the center comes out clean, 45 to 55 minutes. It really does need to be clean, or the center will collapse. Place the cake in the pan on a wire rack to cool:

If Baking in Springform Pan:

• Cool on rack 20 minutes. Then unlatch springform and cool almost completely before serving.

If Baking in Regular Baking Pan:

• Cool completely and cut cake, serving from the pan.
• To serve, cut and plate portions. Heat each piece for about 15-20 seconds (if not still a little warm from oven) in the microwave, or you may reheat the entire cake in a 200°F oven for about 20 minutes. Pour or drizzle heavy cream or thinned sour cream, if using, and add additional fresh blueberries and serve. This makes a very pretty presentation and it's delicious.
• Store leftover coffee cake, loosley covered, in the fridge. Bring to room temperature before serving.

Optional topping ideas:

• The cake is wonderful served at room temperature or heated. Top with whipping cream poured over the top just before serving with additional fresh blueberries. See next step if you'd like to try thinned sour cream.

Making the optional thinned sour cream for pouring on top:

• Simply use a whisk, pouring ¼ cup cold milk over ½ cup sour cream, whisking until smooth. Add more milk if you want the sour cream even more pourable.