Preheat oven to 350°F.
Grease and line an 8-inch cake tin with parchment paper. I used one with a removable base.
In a large mixing bowl add flour, baking powder, baking soda and sugar. Add chopped apple and stir to combine.
In a separate bowl add eggs, vanilla, oil and kefir. Whisk to break up the egg yolks.
Add wet ingredients to dry ingredients and mix with a wooden spoon until just combined. Spoon batter into prepared tin.
Top the batter with sliced apples.