Preheat Oven – Set to 180°C (350°F). Grease and line a small cake pan.
Cream Butter & Sugar – In a bowl, beat butter and sugar until light and fluffy.
Add Eggs & Vanilla – Mix in eggs one at a time. Stir in vanilla extract.
Mix Dry Ingredients – In another bowl, combine flour, baking powder, salt, and optional cinnamon.
Combine Wet & Dry – Gently fold dry mixture into the butter mixture. Add lemon zest.
Add Plums – Pour batter into pan. Arrange plum slices on top.
Bake – Place in oven for 30–35 minutes until golden. Check with a toothpick.
Cool & Serve – Let it rest for 10 minutes. Slice and enjoy warm or at room temperature.