Heat the oven to 400°F. Butter and flour a 9-inch springform pan.
Cream the butter and sugar in a standing mixer until light and fluffy.
On the lowest speed, add the eggs one at a time.
Slowly add the flour, salt, ricotta, lemon zest, baking powder, and grated apple.
Scrape the batter into the prepared pan, smooth the top and bake for 25-30 minutes, until the cake is deep golden brown and the sides start to pull away from the pan.
Cool in the pan on a wire rack for 10 minutes.
Turn the cake out of the pan and cool completely on the rack.
Using a long, serrated knife, slice the cake in two horizontally. Because the cake is tender and will break, use a rimless cookie sheet or the bottom of a French tart pan to slide under the top of the cake to remove it. (See photo.)
Spread the jam over the bottom and then return the top.
Sift confectioners' sugar over the top and serve.