Strawberry Rhubarb Crumb Bars

["Preheat oven to 350u00b0F. Butter a 9"" square baking pan and line with parchment paper, leaving a 2"" overhang on 2 sides. Butter and flour parchment paper and pan, tapping out the excess flour. Set aside.", "For the streusel:", "Whisk together the butter, brown sugar, and salt. Add flour and mix with a fork until large crumbs form. Refrigerate until ready to use.", "For the bars:", "In a medium bowl, combine rhubarb, strawberries, brown sugar, and 1/4 cup of the flour. In another medium bowl, whisk the remaining 1-1/4 cup of flour, baking powder, and salt. In a large bowl, using an electric mixer, beat butter and powdered sugar until light and fluffy. Beat in the eggs, one at a time.", "With the mixer on low, beat in vanilla, then the flour mixture. Spread batter in prepared pan. Top with rhubarb and strawberry mixture, then top with prepared streusel. If you like to have some of the pretty red of the rhubarb and strawberries show on top (I do!), poke a few pieces up through the streusel.", "Bake until golden and a toothpick inserted in the center comes out with just a bit of moist crumbs attached, about 50 to 55 minutes. Let cool completely in pan.", "Run a knife around the edge of the pan and, using the parchment paper overhang, lift cake from pan. Cut into bars and dust with powdered sugar. Serve as is, or with some freshly whipped and sweetened cream-so lovely!", "Source: heavily adapted from Everyday Food magazine, May 2009."]