• Preheat the oven to 350℉/180℃ and line an 8x8 inch pan with parchment paper.

Apple Pie Filling

• Peel, core, and dice the apples into 1cm pieces.
• Add the apples, spices, lemon juice, and sugar to a medium saucepan over medium heat and cook for about 5 minutes until the apples begin to soften slightly.
  2 large tart baking apples, 2 teaspoons ground cinnamon, ¼ teaspoon ground cloves, ¼ teaspoon ground nutmeg, 2 tablespoons lemon juice, 2 tablespoons light brown sugar
• Mix together the cornstarch and water and stir it into the apples. Cook for thirty seconds, stirring the whole time.
  1 tablespoon cornstarch, 2 tablespoons water
• Remove the pan from the heat and stir in the vanilla. Set aside while you make the streusel and cake batter.
  1 teaspoon vanilla paste or extract

Streusel Topping

• Melt the butter, then use a fork to mix it together with the flour, sugar, oats, and salt until medium sized crumbles form. Set aside.
  ¾ cup all-purpose flour, ¾ cup light brown sugar, ½ cup rolled oats, ¼ cup unsalted butter, ¼ teaspoon kosher salt

Cake

• Whisk together the flour, baking powder and salt and set aside.
  1½ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon kosher salt
• In the bowl of a stand mixer, cream together the butter and sugar on medium high speed for five minutes, then scrape down the sides.
  6 tablespoons unsalted butter, 1 cup light brown sugar
• Add the eggs and vanilla and mix on medium speed for one minute. Scrape down the sides again.
  2 large eggs, 2 teaspoons vanilla paste or extract
• Mix together the sour cream and milk. With the mixer running on low, add half of the flour mixture then half of the milk mixture. Repeat with the remaining wet and dry ingredients.
  ¼ cup full-fat sour cream, ¼ cup whole milk
• Mix until the flour is just combined, and use a rubber spatula to finish folding in any flour on the sides of the bowl.

Assembly

• Add the cake batter to the prepared pan and spread it to meet the edges and corners.
• Add the apple pie filling evenly over top of the cake batter.
• Sprinkle the streusel over the top, getting it the whole way to the edges.
• Bake the coffee cake in the center rack of the pre-heated oven for 45-55 minutes until no longer jiggly and a toothpick inserted down into the cake layer comes out clean.
• Place the pan on a wire rack to cool, at least one hour, before slicing.