Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan by greasing it with butter, dusting with flour, and lining with parchment paper.

Peel, core, and dice about two medium Granny Smith apples into 0.5-inch pieces and toss with lemon juice.

In a medium bowl, combine all-purpose flour, baking powder, kosher salt, and ground cinnamon.

In a large bowl, whisk together granulated sugar, light brown sugar, and melted unsalted butter until smooth, then add eggs and vanilla extract.

Mix in sour cream and milk until creamy.

Gently fold the dry ingredients into the wet mixture.

Fold in the diced apples until evenly distributed, then pour the batter into the prepared cake pan.

In a small bowl, mix together granulated sugar and ground cinnamon, then sprinkle over the batter.

Bake the cake for about 45 minutes until golden brown and a toothpick comes out clean.

Allow the cake to cool in the pan for about 15 minutes before transferring it to a wire rack to cool completely.