Preheat your oven to 350 degrees Fahrenheit. Butter an 8-inch square baking pan. Line the pan with parchment paper, leaving a 2-inch overhang on two sides. Butter and flour the parchment and pan, tapping out any excess flour.
In a medium bowl, whisk together melted butter, brown sugar, and salt. Add flour and mix with a fork until large crumbs form. Refrigerate until ready to use.
In a medium bowl, combine rhubarb, strawberries, brown sugar, and 1/4 cup of flour.
In another medium bowl, whisk 3/4 cup of flour, baking powder, and salt.
In a large bowl, using an electric mixer, beat butter and confectioners' sugar until light and fluffy, about 3 minutes.
Beat in eggs, one at a time; turn the mixer to low speed and add in vanilla. Gradually add in the flour mixture, mixing until just combined.
Spread batter in prepared pan. Sprinkle with rhubarb and strawberry mixture and top with streusel.
Bake cake for 45-50 minutes or until the top is golden and a toothpick inserted in center comes out with moist crumbs attached.
Let cool completely in pan. Using paper overhang, lift cake from pan. Cut into 16 bars.
