Preheat oven to 350F. Butter and line a 9" square pan with parchment paper. Set aside.
Prepare the streusel first. In a small bowl, combine the flour and sugar.
1/2 cup (63g) all-purpose flour, 1/4 cup (50g) granulated sugar
Cut in the butter until it resembles coarse crumbs. Set aside.
1/4 cup (57g) unsalted butter
In a large mixing bowl, cream the butter and 1/2 cup sugar until light and fluffy.
1/2 cup (114g) unsalted butter, 1/2 cup (100g) granulated sugar
Mix in the eggs, one at a time, then mix in the vanilla.
2 large eggs, 1 tsp vanilla extract
Add the flour, baking powder, and salt. Stir until just combined. The batter will be thick.
1 cup (125g) all-purpose flour, 1 tsp baking powder, 1/4 tsp salt
Evenly spread the batter into the prepared pan. Arrange the plums on top.
3 cups fresh plums
Mix the cinnamon with the remaining 1 tablespoon of sugar and sprinkle over the plums. Top with streusel.
1 Tbsp granulated sugar, 1/4 tsp cinnamon
Bake for 40-45 minutes or until golden on top and plums are tender.
Notes
Leftovers can be transferred to an airtight container and stored at room temperature for 1-2 days or in the fridge for up to 5 days. Bring to room temperature before serving.