Cream the sugar, butter and vanilla seeds together until pale and fluffy. Stir in the eggs, one at a time, beating after each addition. Stir in the ground almonds (the mixture will be a little stiff, but the milk will loosen the batter). Add half of the milk and stir to incorporate, followed by the baking powder and half of the flour. Mix in the remainder of the milk, finishing with the rest of the flour, gently stirring until smooth and combined. Spoon the batter into the prepared tin and top with the plums, cut-side up, pressing them into the mixture a little.