Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, whisk together the flour, ground almonds, sugar, baking powder, and salt.

In another bowl, cream the softened butter until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Arrange the sliced plums on top of the batter, pressing them in slightly. Sprinkle with brown sugar and cinnamon if using.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.