Preheat the oven to 350 degrees and line an 8x8 pan with parchment paper.
In a bowl, combine yogurt, oil, brown sugar, eggs, and vanilla. Stir until well combined.
Add flour, cinnamon, baking powder, baking soda, and salt to the bowl. Stir until just combined.
Finely chop your apple, avoiding the core but leaving on the skin. Stir the apple pieces into the batter, which will be very thick.
Transfer the batter to the lined baking pan and smooth down the batter. Sprinkle 1 tablespoon of light brown sugar over the top before placing in the oven.
Bake the cake for 45-50 minutes, until a knife comes out clean when inserted into the middle.
Allow the cake to cool for 10 minutes before slicing into squares and serving.
Notes
Leftovers can be stored in an airtight container on the counter for 1 day, or in the fridge for up to 5 days.
Recipe by Uproot Kitchen at https://uprootkitchen.com/2016/11/07/whole-wheat-apple-cake/