Preheat your oven to 350°F (175°C). Grease and flour a 9x9-inch baking pan or line it with parchment paper.
In a bowl, combine the flour, brown sugar, and cinnamon for the streusel. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream together the softened butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream. Begin and end with the dry ingredients, mixing until just combined.
Spread half of the batter into the prepared baking pan. Sprinkle half of the streusel topping over the batter.
Repeat with the remaining batter and streusel. Use a knife to swirl the batter and streusel for a marbled effect.
Bake in the preheated oven for 40-45 minutes or until a toothpick inserted into the center comes out clean.
Allow the coffee cake to cool in the pan for about 15 minutes, then transfer it to a wire rack to cool completely.
Once cooled, slice and serve. Enjoy your moist cinnamon streusel coffee cake!