Heat a skillet over low heat and sprinkle the sugar over the surface. Allow the sugar to melt without stirring until golden brown. Immediately remove the skillet from the heat. Add the butter and mix until combined. Pour the caramel over the bottom of the springform pan. Set aside.

Peel and core the pears, then slice them. The slices should not be too thick or too thin. Arrange the pear slices in a circular pattern over the caramel in the pan. Set aside.

Combine the cake flour, baking powder, ground cinnamon, and salt in a medium bowl. Set aside.

Beat the butter, white sugar, and brown sugar in a large bowl with a hand mixer on medium speed for 3-5 minutes. Slowly add the eggs one at a time and mix well after each addition. Add the vanilla extract and beat until the mixture is pale and fluffy.

Gradually add the flour mixture in batches, alternating with the milk. Mix on low speed until well combined.

Pour the cake batter over the pears in the pan and place the pan in the oven. Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool on a wire rack for 15 minutes. Carefully release and remove the sides of the springform pan. Invert the cake onto a plate, then slowly remove the bottom of the pan.

Slice and serve the upside-down pear cake.