2 green apples, peeled, cored, and thinly sliced (~1/4 centimeter thick)
for the swirl:
3/4 cup brown sugar
1 tablespoon cinnamon
Preheat oven to 375°F. In a small bowl, mix together with a fork the brown sugar and cinnamon. Set aside.
In a large bowl, cream the butter and sugar, then add the eggs, vanilla, and yogurt. In a separate bowl, mix the flour, baking powder and soda, and salt. Once mixed, add the flour to the wet ingredients in batches, making sure to stir between additions.
Butter and flour a large bundt pan. Pour in half of the batter and sprinkle half of the swirlies mixture over it. Using a fork, push in swirlies to integrate them into the batter. Place half of the apple slices over the batter. Repeat with the second half of the batter, sprinkling almost all of the swirlies (leave ~2 tablespoons) over the top and then place the remaining apple slices on top. Sprinkle the remaining swirlies over the apple slices. Bake for about 40 minutes, or until knife comes out clean when inserted into cake. Eat it right out of the oven, or let cool and refrigerate for days!