Preheat oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, combine 1/2 cup butter and granulated sugar. Beat the eggs one at a time, and then mix in the vanilla. Mix in sour cream.
Gradually add flour mixture to wet ingredients, mixing until combined.
Pour half of the batter into the prepared pan.
In a small bowl, combine brown sugar, cinnamon, and 1/4 cup cubed butter to make the streusel. Sprinkle half over the batter in the pan.
Spread the remaining dough over the streusel layer, then top with the remaining streusel.
Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Preparation time: 20 minutes | Cooking time: 40 minutes | Total time: 1 hour
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