To make the crumble topping: In a medium bowl, add the almond flour, butter, cinnamon, and brown sugar. Cut the butter into the dry ingredient with a fork or your fingers, until you get a crumbly mixture. Set aside.

To make the pear filling: In a small bowl, add the pear cut into slices and stir in a small bowl with the cinnamon and brown sugar. Set aside.

Preheat the oven to 375F. Line the bottom with parchment paper and grease the side with butter (or spray) of an 8-inch (20cm) round springform cake pan.

To make the cake batter: Combine the butter and sugar in a large mixing bowl and beat with a hand mixer until creamed. Add the egg yolks, vanilla extract, and yogurt and beat until smooth. Add the almond flour, potato (or corn) starch, baking powder, and salt. Mix at low speed until just combined.

Beat the egg whites to stiff peaks, and using a spatula, gently fold about a third of the whipped egg whites into the cake batter. Add the rest of the whipped egg whites in 2 times, gently folding it into the batter. Don't overwork the batter or it will deflate the egg whites.

Spread half of the batter into the prepared pan. Top with a single layer of the prepared pear slices (don't add the juice if any). Spoon the remaining batter on top and carefully spread into one layer.

Top with the prepared crumble and almond slices. Peel and core the remaining large pear, cut into quarters, and slice each quarter. Arrange the sliced pear on top and press into the crumble.

Bake in the middle of the oven for about 55-65 minutes, or until a toothpick inserted in the center of the cake comes out clean. Check after about 40 minutes, and cover the top loosely with aluminum foil if the top starts to darken too much. Let the cake cool completely before removing from the pan. Sprinkle with powdered sugar if desired and serve!