Heat the oil in a non-stick frying pan over high heat. Add the mushrooms and chopped rosemary. Cook for 5 minutes or until the mushrooms are tender. Set aside to cool for 10 minutes.
Meanwhile, preheat oven to 200°C fan forced. Lightly grease three, 12 hole, 30ml-capacity mini muffin pans with olive oil spray.
Sift the flour and baking powder into a bowl. Add the parmesan and season with salt and pepper. Using a fork, whisk the eggs and buttermilk together in a jug. Pour the egg mixture into the dry ingredients. Add the butter and stir until almost combined. Fold in the mushrooms.
Spoon mixture into muffin holes until almost full. Bake for 18–20 minutes or until a skewer inserted into a muffin comes out clean. Stand muffins in pan for 5 minutes then turn out onto a wire rack.
Cut rosemary sprigs into 3cm lengths. Top muffins with a piece of salmon and secure with a rosemary sprig. Serve.