With the rack in the middle position, preheat the oven to 350°F (180°C).
In a bowl, combine all of the ingredients and mix until the crumble holds its shape when pressed. Set aside.
In a bowl, mix the plums together with 2 tbsp of the sugar. Divide between four small baking dishes or ramekins about 5 inches (13 cm) in diameter.
In the same bowl, mix the flour with the baking powder.
In another bowl, cream the butter with the remaining sugar using an electric mixer. Add the egg and beat until well combined. On low speed, add the dry ingredients. Place the batter on top of the plums, spreading it carefully with the back of a spoon. Sprinkle the crumble on top. Place the baking dishes on a baking sheet.
Bake for 30 minutes or until the batter is cooked through but still springy. Let cool for 10 minutes before serving. Delicious with yogurt.