• Preheat the oven to 350℉/176℃. Line a 9x9 pan with parchment paper and set aside. Slice the plums into quarters and set aside.

For the streusel

• Combine softened butter, flour, and white sugar with your hands or a fork until a crumbly streusel forms. Place in the fridge to chill.
  2 tbsp unsalted butter, 1/2 cup all-purpose flour, 3 tbsp white sugar

For the cake

• In a bowl, add the flour, baking powder, and salt. Stir together and set aside.
  1 3/4 cups all-purpose flour, 1 1/4 tsps baking powder, 1/4 tsp salt
• In another bowl, add the butter, lemon zest, and white sugar. Beat on medium-high speed for 2-3 minutes or until light and fluffy, scraping the sides of the bowl as needed.
  3/4 cups unsalted butter, 1/2 cup white sugar, Lemon zest from 1 small lemon
• Add in one egg at a time, beating very well between each addition (about a minute in between each addition). Scrap the sides of the bowl as needed. Then add in the lemon juice and vanilla, and continue beating until combined.
  3 large eggs, Lemon juice from 1 small lemon, 1 tbsp vanilla
• Add in the dry ingredients. With a spatula, gently fold just until a smooth cake batter forms and no dry streaks appear. Do not over-mix!
• Transfer and spread evenly into the prepared pan. Add the plums evenly on top. Finish with sprinkling the streusel.
• Bake for 45-60 minutes. The cake will be ready when a toothpick inserted comes out clean. Allow to cool, slice, and enjoy!