Preheat your oven to 350°F (175°C). Line a 9-inch square baking pan with parchment paper or spray it with cooking spray.

In a large mixing bowl, whisk together the flour, baking powder, and salt until evenly combined.

In a medium mixing bowl, cream the softened butter and granulated sugar with a whisk or wooden spoon for about 2 minutes until light and fluffy.

Add the egg and vanilla extract to the butter mixture and stir vigorously for about 30 seconds until smooth.

Alternate adding the flour mixture and sour cream to the butter mixture in three parts, beginning and ending with flour. Gently fold until just combined without overmixing.

In a small bowl, combine the streusel topping ingredients and rub in the cold cubed butter with your fingers until the mixture forms coarse crumbs.

Spread the cake batter evenly in the prepared pan and sprinkle the streusel topping evenly over the surface, pressing lightly to adhere.

Bake for 38 to 42 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with a few moist crumbs.

Let the cake cool in the pan for 10 minutes, then run a knife around the edges, lift it out using the parchment, and transfer to a wire rack to cool.