Preheat oven to 350º. Butter 8×8-inch pan.
Prepare stone fruit: Wash and cut in half, removing pits. If fruit is small, cut each half in quarters. If it’s large, cut each half in 3-4 thin wedges. Set aside.
Combine flour, baking powder and salt in a bowl. Set aside.
In mixing bowl, cream butter and sugar until fluffy. Add eggs, one at a time, beating well after each addition. Add one-third of flour mixture, beating just until blended. Add half of sour cream and mix till blended. Repeat, ending with flour. Don’t overbeat. Spread batter in pan. Arrange fruit slices in even rows on top, flesh side up.
Sprinkle lemon juice on fruits. Combine sugar and cinnamon and sprinkle evenly atop all fruits. Dot with butter.
Bake 50-55 minutes, until top is bubbling and toothpick inserted in cake part comes out clean. Cool on rack. Cut into squares to serve.