Preheat your oven to 375°F (190°C). Grease and flour a 9-inch round cake pan.

Prepare the plums: In a small bowl, toss the plum halves with melted butter, honey, and cinnamon until evenly coated. Set aside.

Whisk dry ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Cream butter and sugar: In a large bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy, about 3 minutes. Beat in the eggs one at a time, followed by the vanilla extract.

Combine wet and dry mixtures: Gradually add the flour mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the flour. Mix until just combined, taking care not to overmix.

Assemble and bake: Pour the batter evenly into the prepared cake pan. Arrange the honey-roasted plum halves cut-side up decoratively over the top of the batter. Drizzle any remaining honey mixture over the top.

Bake for 50 to 55 minutes, or until a toothpick inserted into the center of the cake (avoiding a plum) comes out clean. Let cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.