FOR THE STREUSEL
6 T. unsalted butter, melted plus room temperature for pan
1 Cup Flour, plus more for pan
½ Cup packed brown sugar
¼ tsp. salt
FOR THE CAKE
½ # rhubarb, cut into ½ inch pieces
1 T. brown sugar
1 Cup Flour
¼ tsp. salt
½ tsp. Baking powder
½ Cup unsalted Butter, room temperature
1 Cup powdered sugar
2 large Eggs
½ tsp. Vanilla
Preheat oven to 350º
Butter an 8 inch square baking pan. Line with parchment paper, leaving a 2inch overhang on 2 sides. Butter and flour the parchment in pan, tapping out excess flour.
Make streusel: whisk together 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, brown sugar, and 1/4 cup flour. In another medium bowl, whisk 3/4 cup flour, baking powder, and salt. In a large bowl, using an electric mixer, beat butter and powdered sugar until light and fluffy; beat in eggs, one at a time. With mixer on low, beat in vanilla, then flour mixture. Spread batter in prepared pan. Sprinkle with streusel.
Bake cake 45-50 minutes, until golden and a toothpick inserted comes out with moist crumbs attached. Let cool completely in pan. Using paper overhang, lift cake from pan.
Cut into 16 bars.