Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon (if using).
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Begin and end with the dry ingredients, mixing until just combined.
Gently fold in the fresh cranberries, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared cake pan, smoothing the top with a spatula.
Bake in the preheated oven for 45 minutes, or until a toothpick inserted in the center comes out clean.
Remove the cake from the oven and let it cool in the pan for about 10 minutes. Then transfer it to a wire rack to cool completely.
Once cooled, slice, serve, and enjoy!