Preheat oven to 350°F and grease a 9x13 inch baking dish.

In a large bowl, whisk together flour, baking powder, and salt.

In another bowl, beat softened butter and sugar until fluffy.

Add eggs one at a time, beating well after each addition.

Mix in sour cream and vanilla extract until smooth.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Fold in chopped apples.

Pour batter into the prepared baking dish.

Make the streusel topping by mixing flour, brown sugar, cinnamon, and melted butter until crumbly.

Sprinkle streusel evenly over the batter.

Bake for 50-55 minutes or until a toothpick inserted into the center comes out clean.

Allow to cool before serving.