Preheat toaster oven to 300 degrees. Line the bottom of a 5 x 7 baking dish with parchment paper and lightly spray the dish with oil.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt.
In a large bowl, whisk together the butter, sugar, sour cream, and egg, about 2 minutes. Stir in the vanilla and lemon juice.
Slowly add the dry ingredients to the wet ingredient until just combined.
Dice the apples and sprinkle with the cinnamon then fold the apples into the batter.
Pour the batter into the prepared pan and tap the pan to even out the batter.
Bake on the lower rack of a toaster oven for 45 minutes or until a cake tester inserted into the center comes out clean.
Remove from the oven and let cool before removing from pan.
Remove from pan and lightly dust with the confectioner sugar.
Slice into six smaller or four larger servings.
Notes
Choose the Right Apples: You need apples that hold up under heat like Granny Smith, Honeycrisp, or Braeburn apples. Proper Preparation of Apples: Peel, core, and slice your apples properly. Thin, even slices will cook uniformly, contributing to a more consistent flavor and texture.Spice Mix: An apple cake isn't complete without the warming flavors of spices like cinnamon and nutmeg. Some recipes might also include allspice or cloves. Be sure to blend these spices thoroughly into your dry ingredients to ensure even distribution in your batter.Balancing Moisture: Apple cakes should be moist but not soggy. If your apples are excessively juicy, consider tossing them in a bit of flour before adding them to the batter. This can help absorb some of the excess liquid.Don't Overmix: Overmixing can result in a tough, dense cake. The wet and dry ingredients should be just incorporated, and there should be no large pockets or streaks of flour, but a few small lumps are okay.Baking Time: To avoid overbaking, check for doneness about 10 minutes before the recommended baking time ends.Cooling: Cool in the pan on a wire rack for about 10-15 minutes before removing. It may fall apart if you try to remove it immediately after baking.