Preheat the oven 350 degrees F (180 degrees C).
Grease and flour a 10-inch round cake pan. Line the bottom of the pan with parchment paper. Spray the top of the parchment paper with cooking spray or use butter/oil and flour to grease the pan.
Vigorously whisk Egg (6) and Granulated Sugar (1 cup) together until they are pale yellow and foamy.
Add the Sour Cream (1/4 cup) to the batter. Dissolve the Baking Soda (3/4 teaspoon) in Distilled White Vinegar (1/2 teaspoon) and add to the batter also. Mix to combine.
Add the All-Purpose Flour (1 1/2 cup) to the batter and mix just until the flour is incorporated.
Meanwhile, peel the Granny Smith Apple (5) and cut into small, thin pieces. Add the apples to the prepared cake pan.
Pour the batter over the apples.
Distribute the batter evenly over the apples.
Bake in the preheated 350 degrees F (180 degrees C) oven for 45-50 minutes, until the Sharlotka is golden, or when a toothpick comes out clean from the center of the cake.
Run an offset spatula or butter knife around the edges of the cake pan. Flip the cake over onto a cooling rack. The parchment paper will peel right off.
Flip it over right side up. Allow it to cool for about 15 minutes. Place the Sharlotka onto a serving platter. Dust with confectioner's sugar. You can also sprinkle it with a bit of cinnamon if you like.