Preheat your oven to 350°F and lightly grease a 7x5-inch baking dish. If you’d like to lift the cake out of the dish later, lining it with parchment paper is a great idea.
In a small bowl, mix together the diced Granny Smith apples, granulated sugar, lemon juice, and ground cinnamon. Stir these together gently and set them aside. You want the apples to soak up those flavors while you prepare the cake batter.
In another small bowl, combine the softened butter, brown sugar, all-purpose flour, and ground cinnamon. Use a fork to mix these ingredients until they resemble a thick paste. Set aside as this will create a lovely swirl in your cake.
Next, whisk together the remaining all-purpose flour, baking powder, and salt in a small bowl. This will ensure that everything is evenly distributed.
In a medium bowl, cream together the softened butter and granulated sugar until light and fluffy. You can use a hand mixer or a whisk for this step. It should take a few minutes.
Add the room temperature egg and vanilla extract to the mixture, beating until well combined. Then, add the sour cream and one tablespoon of the milk, mixing until smooth. This will give your cake a nice moist texture.
Now, add half of the flour mixture to the wet ingredients and mix until just combined. Be careful not to overmix, as this can make the cake tough.
Repeat this with the remaining tablespoon of milk and the second half of the flour mixture until you have a smooth batter.
Transfer the batter to your prepared baking dish and smooth out the top with a spatula.
Take the apple mixture and the cinnamon swirl mixture you prepared earlier. Drop spoonfuls of this mix over the top of your cake, spreading it as evenly as you can. Use a knife to swirl the apple mixture into the batter. Make sure to cut deeply into the batter but avoid completely mixing the two together.
Bake in the preheated oven for 27 to 35 minutes. You’ll know it’s done when a toothpick inserted into the center comes out with just a few moist crumbs.
While your cake is still warm, whisk together all the glaze ingredients in a small bowl until smooth. Pour this over your cake, allowing it to soak in a bit. Let the glaze set for 15 to 20 minutes before serving.
Slice, serve, and enjoy your delicious Small Apple Cinnamon Cake!
