In a large mixing bowl, combine 2 cups of all-purpose flour, 1 cup of granulated sugar, 1 teaspoon of baking powder, 1 teaspoon of baking soda, ½ teaspoon of salt, and 1 teaspoon of ground cinnamon. Whisk together until the mixture is well blended, ensuring there are no lumps.

In a separate bowl, whisk together 1 cup of sour cream, 2 large eggs, ½ cup of melted unsalted butter, and 2 teaspoons of vanilla extract until the mixture is completely smooth.

Gently fold the wet mixture into the dry ingredients using a spatula or wooden spoon until just combined.

In a small bowl, mix together ½ cup of additional flour, ⅓ cup of sugar, 1 teaspoon of ground cinnamon, and 3 tablespoons of softened butter until you achieve a sandy, crumbly texture.

Preheat your oven to 350°F (175°C). Grease a baking dish and pour half of the cake batter into the bottom. Sprinkle half of the prepared streusel topping over the batter, then pour in the remaining batter and top with the rest of the streusel.

Place the baking dish in the preheated oven and bake the coffee cake for about 50 minutes. Let it cool slightly before serving.