Preheat the oven to 400°F (200°C).

Grease an 8" x 8" baking dish with butter and dust with flour; set aside.

In a medium bowl, whisk together the flour, 2 tbsp.

sugar, baking powder, and kosher salt.

Add 4 tbsp.

chilled butter and rub into the flour mixture until pea-size pieces form.

In a small bowl, mix together the milk, vanilla extract, and egg.

Add this mixture to the flour mixture and stir with a wooden spoon until just combined.

Transfer the dough to the prepared baking dish and spread it over the bottom of the dish.

Arrange the plum slices in rows on top of the dough.

Combine the remaining ½ cup of sugar and ground cinnamon in a small bowl and sprinkle over the plums.

Melt the remaining 3 tbsp.

of butter and drizzle over the plums.

Bake until browned and bubbly, about 30 minutes.

Let cool slightly before slicing and serving.