Combine sugar and butter in a large bowl and beat with an electric mixer until light and fluffy. Add eggs, 1 at a time, and beat well after each addition. Add vanilla extract along with plain yogurt and mix well.

Combine flour, baking powder, and salt in a separate bowl and sift on top of the butter mixture add ground almonds mix to combine. Transfer batter into the prepared springform pan and smooth out.

Set plums inside batter, lengthwise and sprinkle evenly with demerara sugar.