Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan or a 9x9 inch square baking dish with butter or non-stick spray.
In a small bowl, combine the brown sugar, flour, and cinnamon. Add the cold cubed butter and use a fork or pastry cutter to mix until the mixture resembles coarse crumbs. Set aside.
In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy.
Mix in the eggs one at a time, ensuring each is incorporated well. Then, add the sour cream and vanilla extract, mixing until smooth.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Gradually add the dry ingredients to the wet mixture, stirring just until combined.
Pour half of the cake batter into the prepared pan and smooth it out. Sprinkle half of the streusel topping over the batter. Pour the remaining batter on top, then sprinkle the rest of the streusel evenly over the cake.
Bake in the preheated oven for about 40-50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely. Slice and serve.
Notes
Serve warm or at room temperature. Great with coffee!