Preheat oven to 160°C (approximately 320°F.) Butter loaf pan and sprinkle with flour.
Rinse, pit and quarter plums. Rinse lemon in hot water, pat dry, zest and squeeze juice.
Mix flour and baking powder in bowl. Cut butter into pieces and melt in small pot. Remove pot from heat and let butter cool. Beat eggs, sugar and vanilla sugar until foamy. Mix lemon juice and zest with eggs then alternatingly stir melted butter and flour mixture into eggs to form smooth batter.
Pour about 1/3 batter into greased loaf pan, smooth surface and top with 2/3 plums. Cover plums with remaining batter, sprinkle batter with remaining plums and bake about 50 minutes in preheated oven until bread is golden brown and toothpick inserted in center comes out clean.
Remove from oven, let rest briefly, separate bread from loaf pan and set on wire rack to cool.