For the topping
1 stick unsalted butter
1/2 cup all-purpose flour
1/4 cup whole wheat pastry flour
1/4 teaspoon baking powder
1/8 teaspoon salt
3 tablespoons ground almonds, see note
1/2 cup fine grained raw cane sugar
1 egg yolk
1 teaspoon vanilla extract
For the filling
1 tablespoon cornstarch
1/4 cup light brown sugar packed, or more, depending on your fruit
1/2 teaspoon ground cinnamon
1/4 teaspoon ground ginger
A pinch of salt
1 1/2 pounds plums, pitted, halved if small, quartered if large
In a small bowl, whisk together the flours, baking powder, salt and ground almonds. Set aside.
In a small saucepan over medium-high heat, melt the butter, swirling occasionally. Once the butter has melted, continue to swirl the pot, as the butter begins to darken and brown. When the butter is amber in colour and aromatic, remove it from the heat and pour into a medium heat safe bowl to cool slightly. Pour in the sugar, and beat with a wooden spoon until the mixture lightens in colour. Stir in the egg yolk and vanilla. Add the flours and stir until combined. Refrigerate the dough and preheat an oven to 375°F (190°C).
Combine the cornstarch with the brown sugar, spices and salt in a medium bowl. Add the plums and toss gently to coat.
Tumble the plums into an 8-inch square baking dish. Drop the dough by heaped spoonfuls over the fruit, covering evenly. Bake in the preheated oven until the juices are bubbling and the topping is golden brown, about 40-45 minutes. Let stand to cool slightly before serving.