Preheat your oven to 350°F (175°C) and grease a bundt or round cake pan.
In a bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
In another bowl, cream together softened butter and granulated sugar until light and fluffy. Add eggs one at a time, then mix in sour cream and vanilla until smooth.
Gradually incorporate the dry mixture into the wet ingredients until just combined.
Prepare the streusel by mixing brown sugar, flour, melted butter, and additional cinnamon until crumbly.
Pour half of the batter into the prepared pan, sprinkle half of the streusel over it, add the remaining batter, and top with the rest of the streusel.
Bake for 40-50 minutes or until a toothpick comes out clean. Let cool before serving.
