• Preheat oven to 350°F and grease or line an 8x8-inch or 9x9-inch square baking pan with parchment paper; set aside.

Make the Streusel Topping

• In a small bowl, combine the flour, brown sugar and cinnamon.
  2/3 cup flour, 1/2 cup brown sugar, 1 teaspoon cinnamon
• Add the softened butter and use a fork to cut the butter into the dry ingredients until the mixture begins clumping together; set aside.
  4 tablespoons butter

Make the Batter

• Add the flour, baking powder, baking soda and salt to a bowl; stir and set aside.
  1 3/4 cup flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
• Add the butter and sugars to a large mixing bowl or a the bowl of a stand mixer.
  6 tablespoons butter, 1/3 cup sugar, 1/4 cup brown sugar
• Cream the butter and sugars for a couple minutes, or until light and fluffy.
• Add the egg and vanilla; stir well, scraping down the sides at least once.
  1 egg, 1 teaspoon vanilla
• Add about half of the dry ingredients and mix to combine.
• Continue mixing while adding the buttermilk slowly; scrape the sides of the bowl.
  3/4 cup buttermilk*
• Add the remaining dry ingredients, stirring until just combined.

Make the Apple Filling

• Combine the brown sugar, flour and cinnamon in a bowl.
  1/2 cup brown sugar, 1 1/2 tablespoons flour, 1 1/2 teaspoon cinnamon
• Add the peeled and diced apples and stir until well-combined; set aside.**
  1 1/2 cups diced apples

Assemble and Bake

• Pour a little more than half of the batter into the prepared pan; spread to an even layer.
• Sprinkle the apples over the batter.
• Spoon the remaining batter over the apples and smooth it out to an even layer.
• Top with the streusel topping, pressing it into the batter gently.
• Bake for about 30 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs (not runny batter).
• Let cool, then cut and enjoy!