In a mixing bowl, beat butter and sugar until light and fluffy. Beat in the yogurt.
Combine the flour and baking powder; add to the creamed mixture alternating with the buttermilk, beating well after each addition. Add lemon peel.
Pour into a greased 8-inch round baking pan (or use 8-inch spring form pan).
Arrange the plums on the top. Gently press into batter. Sprinkle with the brown sugar and cinnamon.
Bake in a preheated oven at 350 F/180 C for 40 minutes or until a toothpick inserted near the center of the cake comes out clean.
Remove from oven and cool. Run a knife around edge of the pan; and cut in slices to serve.