Preheat the oven to 375 degrees. Butter the inside of a 9-inch round cake pan, then line with parchment paper. The butter will help the paper adhere.
Cream the butter and sugar together using an electric mixer until light and fluffy, about 3 minutes on medium speed.
Scrape down the bowl, then add the eggs one at a time, being sure to fully incorporate the first before adding in the next.
Scrape down the bowl again, then add the ricotta, sour cream, vanilla, orange zest, rosemary and salt, then mix together. The mixture will be lumpy from the ricotta - this is normal!
Add the flour and baking powder, then mix on low speed until just combined. Don't over mix!
Pour the batter into the prepared pan and smooth the top. Arrange the pear slices on top in an artful pattern, then sprinkle all over with Demerara sugar.
Bake in the center of the oven for about 50 minutes, or until he top is golden and a toothpick inserted in the center comes out without any raw batter.
Let cool for 30 minutes in the pan, then use the parchment to lift the cake out and transfer to a serving platter. Hold the cake with one hand while you pull the parchment off of the bottom.
Serve warm or at room temperature with a dollop of whipped cream. Keep any leftovers refrigerated.