• Preheat the oven to 350°F.
• Line a 9" x 13" pan with parchment.

Make the streusel topping

• In a food processor, pulse together sugar, salt, flour, and cinnamon.
• Add the butter, and process until well combined.
• Set the topping aside.

Make the filling

• Peel and dice the apples to equal sizes.
• To a medium saucepan, add the apples, brown sugar, cinnamon, butter, and lemon juice.
• Cook over medium heat, intermittently stirring, until the apples soften and the sauce thickens.
• Set aside to cool.

Make the Cake

• In the bowl of a stand mixer, cream the butter with the sugar.
• Add the salt, baking powder, and vanilla and combine.
• Add the eggs one at a time, beating well after each addition.
• Add the sour cream and milk, and mix to combine.
• Add the flour to the mixture, and mix until completely combined and the batter becomes thick.
• Spread ½ of the batter into the prepared pan, all the way to the edges.
• Spread the apple filling on top of the batter, again, all the way to the edges.
• Cover the filling with the remaining batter, continuing all the way to the edges.
• Sprinkle the topping over the batter in the pan.
• Bake the cake for about 60 minutes, until a tester put into the center of the cake comes out clean.
• Remove the cake from the oven.
• Allow to cool for at least 30 minutes before cutting and serving.