Preheat and Prepare: Start by preheating your oven to 350°F (175°C). Grease and flour a 9-inch square baking pan to ensure the cake doesn’t stick.

Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt.

Cream Butter and Sugar: In a large mixing bowl, cream together softened butter and granulated sugar until light and fluffy.

Add Eggs and Vanilla: Beat in the eggs one at a time and then mix in the vanilla extract.

Combine with Sour Cream: Stir in the sour cream until well incorporated.

Combine Wet and Dry Mixes: Gradually add the flour mixture to the wet ingredients, mixing just until combined.

Layer the Batter: Pour half of the batter into your prepared baking pan.

Make Streusel Topping: In a small bowl, mix the brown sugar, cinnamon, and the cold cubed butter until clumpy; this will create the streusel. Sprinkle half of this mixture over the batter in the pan.

Add Remaining Batter: Spread the remaining batter over the streusel layer in the pan, then top with the remaining streusel.

Bake: Place it in the oven and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.

Cool and Serve: Let it cool for a few minutes before slicing and serving. Enjoy!