Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper to prevent sticking.

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy, about 3-5 minutes. This incorporates air into the mixture, helping the cake rise.

Add the eggs one at a time, mixing well after each addition. This ensures that the eggs are fully incorporated and helps create a smooth batter.

In a separate bowl, whisk together the sifted flour, baking powder, baking soda, salt, and ground cinnamon. This helps to evenly distribute the leavening agents and spices.

Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk and vanilla extract. Start and end with the dry ingredients. Mix until just combined; do not overmix to keep the cake tender.

In a bowl, toss the sliced plums with brown sugar and ground cinnamon. This will enhance their natural sweetness and add flavor.

Pour half of the cake batter into the prepared cake pan, spreading it evenly. Layer the cinnamon-sugar plums on top, then pour the remaining batter over the plums, smoothing the top.

Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean. This ensures that the cake is fully cooked through.

Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a cooling rack to cool completely.

Once cooled, slice the plum cake and serve with whipped cream if desired. Enjoy your delicious plum cake!