Preheat the oven to 350 degrees F. Grease a 9-inch cake pan, if using a silicone pan. Put parchment paper on the bottom if using a metal pan.
To make topping, combine butter and brown sugar and pour into a small sauce pan.
Heat over stove until butter is melted, stirring frequently. Turn off the heat and stir in the cinnamon.
Pour brown sugar mixture into cake line (make sure the parchment is in place).
Arrange the apples and nuts over the brown sugar mixture, overlapping tightly and making two layers. (The nuts can be arranged in the center or between the apple slices). Set aside.
To make cake, cream the butter and sugar until light and fluffy.
Add 1 egg and beat until just combined.
Mix in flour, milk, yogurt, and vanilla.
Add the baking powder, salt and cinnamon and mix.
Pour the cake mixture evenly over the apples in the pan.
Put the pan in the oven and bake for 45 or 50 minutes, or until a toothpick inserted in the center of the cake comes out clean. Let sit for about 5 minutes.
Loosen the sides of the cake with a knife. Turn upside down onto a flat serving platter. (The pan should still be warm when you do this, so be careful during this step).
Allow cake to finish cooling before slicing.