Step 1: Prepare the Equipment

1. Preheat your oven to 350°F (175°C). Grease a 9-inch round or square cake pan and line the bottom with parchment paper for easy removal.

Step 2: Prepare the Dry Ingredients

1. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon (if using). Set aside.

Step 3: Cream the Butter and Sugar

1. In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar on medium speed until light and fluffy, about 2-3 minutes.
2. Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Step 4: Combine Wet and Dry Ingredients

1. Gradually add the dry ingredients to the wet mixture in three additions, alternating with the milk or buttermilk. Start and end with the dry ingredients, mixing just until combined. Avoid overmixing, as this can make the cake dense.

Step 5: Assemble the Cake

1. Pour the batter into the prepared pan, spreading it evenly with a spatula.
2. Arrange the halved plums cut-side up on top of the batter, gently pressing them in. Sprinkle 1 tablespoon of sugar over the plums for added sweetness and caramelization during baking.

Step 6: Bake the Cake

1. Bake in the preheated oven for 40-50 minutes, or until a toothpick inserted into the center comes out clean.
2. If the top begins to brown too quickly, cover the cake loosely with aluminum foil during the last 10 minutes of baking.

Step 7: Cool and Serve

1. Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
2. Dust the cooled cake with powdered sugar before serving, if desired. Enjoy plain or with whipped cream or ice cream for an indulgent treat.