Heat oven to 350° F. Butter a 10-inch tart pan and set aside. In the large bowl of an electric mixer, cream the butter and sugar at high speed. Lower to medium and add the egg. Combine the flour and baking powder, then gradually add to the butter mixture. Mix until fully incorporated. The dough will be very soft. With floured fingers, push the dough into the tart pan to form an even crust. Arrange the apples in concentric circles, then sprinkle with the cinnamon and the remaining sugar. Bake for 50 minutes or until the crust has browned. Let cool. Meanwhile, in a small saucepan on medium-low heat, combine the apricot jam and lemon juice, mixing them into a thin liquid. Remove from heat and brush gently over the entire kuchen. Serve plain or with vanilla ice cream.