Preheat oven to 400 degrees F (205 degrees C) and place rack in center of oven. Butter and flour one - 8 inch (20 cm) spring form pan. (Alternatively spray pan with non stick vegetable/flour spray.) Line the bottom of the pan with parchment paper. Set aside.

In a large bowl, stir or whisk together the flour, ground almonds, baking powder and salt.

In another large bowl, stir or whisk together the oil, yogurt, orange juice and zest, egg, and vanilla extract. Stir in the sugar. Add the wet ingredients to the flour mixture. Stir until just combined. Scrape the batter into the prepared pan and evenly arrange the halved plums on top of the batter, flesh side facing up. Sprinkle with the 1 1/2 tablespoons brown sugar.

Bake in preheated oven for about 35 to 45 minutes or until a sharp knife inserted in the center of the cake comes out clean. Remove from oven and place on a wire rack to cool.

Makes one - 8 inch (20 cm) cake.