Preheat the oven to 350°F / 175°C. Liberally grease a 9-inch round cake pan or cast-iron skillet with half of the melted butter.
Sprinkle the brown sugar and rosemary evenly over the bottom.
Arrange the plum chunks in a single, even layer over the sugar and set the pan aside.
In a large bowl, whisk the remaining melted butter, buttermilk, eggs, and granulated sugar together until light and foamy.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.
Gradually add the dry to the wet , stirring with a spatula only until the batter is well incorporated. Be careful not to overmix; a few lumps are okay.
Carefully pour and spread the batter over the plums in the pan, using the spatula to ensure it's evenly distributed.
Bake for 50 to 60 minutes, or until the top of the cake is golden brown and a toothpick inserted into the center comes out clean.
Let the cake cool in the pan on a wire rack for exactly 5 minutes. Run a thin knife around the edge of the pan to loosen the cake.
Place a large serving plate on top of the cake pan. With confidence and oven mitts, quickly flip the pan and plate together. Lift the pan away; the cake should release onto the plate. If any fruit sticks to the pan, simply lift it off and place it back on the cake.
Serve warm, perhaps with a scoop of vanilla ice cream.
Seasonal Swaps: No plums? This cake is fantastic with sliced apples, pears, blackberries, or pitted cherries. For a Pear and Almond version, swap rosemary for 1/4 cup (960 ml) chopped almonds and half the flour for 1/2 cup (480 ml) ground almonds.
Don't Overmix: Stir the batter only until the flour is just incorporated. A few lumps are okay and are the secret to a more tender cake.
