Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour an 8 or 9-inch round cake pan.
Prepare Plums: Toss the plum slices with lemon juice and set aside.
Mix Dry Ingredients: In a medium bowl, whisk together flour, baking powder, and salt.
Cream Butter and Sugar: In a large bowl, cream the butter, granulated sugar, and brown sugar until light and fluffy.
Add Eggs and Vanilla: Beat in the eggs one at a time, then add vanilla extract.
Combine: Gradually add the dry ingredients to the wet mixture, alternating with milk, starting and ending with dry ingredients. Mix until just combined.
Arrange Plums: Pour the batter into the prepared pan. Arrange the plum slices on top of the batter in a circular pattern.
Sprinkle Sugar: Sprinkle the plums with 1 tbsp of granulated sugar.
Bake: Bake for 45-50 minutes or until a toothpick inserted into the center comes out clean.
Cool:
Allow the cake to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.