• Preheat oven to 350℉. Butter and flour (1) 8x8-inch cake pan. Add a piece of parchment paper across the bottom, leaving a 2-inch overhang on two of the sides. Set aside.

Make the streusel:

• In a medium bowl, stir together the flour, brown sugar, cinnamon, and salt until well combined. Add the butter and mix with a fork until large clumps form. Refrigerate until ready to use.
  1/2 cup packed light brown sugar, ½ teaspoon cinnamon, 1/4 teaspoon fine sea salt, 6 tablespoons unsalted butter, 1 cup all-purpose flour

Make the cake:

• In a medium bowl, combine rhubarb, strawberries, brown sugar, and ¼ cup of the flour until well combined. Set aside.
  1 cup chopped rhubarb, 1 cup chopped fresh strawberries, 1 tablespoon light brown sugar, 0.25 cup all-purpose flour
• In a separate bowl, whisk together the remaining ¾ cup flour, baking powder, and salt. Set aside.
  0.75 cup all-purpose flour, 1/2 teaspoon baking powder, 1/4 teaspoon fine sea salt
• In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl with an electric hand mixer, beat butter and confectioners’ sugar until light and fluffy, about 3 minutes. Beat in the eggs, one at a time, followed by the vanilla, scraping down the sides of the bowl as needed.
  1/2 cup unsalted butter, 1 cup confectioners' sugar, 2 large eggs, 1 teaspoon pure vanilla extract
• With the mixer on low, gradually add in the flour mixture, mixing just to combine.
• Spread batter in the prepared pan. Evenly top with the rhubarb and strawberry mixture, then evenly sprinkle the streusel over the top.
• Bake for 45-50 minutes, or until the top is golden and a toothpick inserted in the center comes out clean with a few moist crumbs attached.
• Let the cake cool completely in the pan. Using the parchment overhang, lift the cake from the pan to slice into squares for serving.