Plum Streusel Cupcakes

2 1/4 cups all purpose flour
3/4 cup granulated sugar
3/4 cup butter
1/2 tsp baking powder
1/2 tsp baking soda
1 cup buttermilk
1 large egg
1/2 tsp pure vanilla extract
1 pound frozen plums
2 tbs brown sugar
In a small pot, stew plums with brown sugar until softened. Remove fruit with a slotted spoon and continue simmering liquid to a syrupy consistency. Add back to fruit.
In a large bowl, whisk together flour and sugar. Thoroughly cut in butter using a pastry blender, until mixture resembles coarse crumbs. Set aside 2/3 cup of crumbs. Into the remaining crumbs, combine baking powder and soda. Make a well in the centre.
In a small bowl, whisk egg and vanilla into buttermilk. Add to dry mixture and bring together with a fork until just moistened (muffin consistency).
Prepare or line a 12 muffin tray. Fill each cup half way with batter. Divide fruit evenly between cups, then top with remaining batter. Mound with streusel. Bake in a 350F oven for 25 minutes, until a tester inserted in the centre of the cupcakes comes out clean. Place tray on a rack to cool at least 5 minutes before un-cupping to serve warm. If not serving warm, remove cupcakes from tin and cool completely.