FOR THE CRUMB BAR BASE:
Using an electric mixer, beat butter with sugar until light and fluffy.
Beat in eggs, one at a time (one minute between each egg addition).
Add vanilla extract.
Turn mixer to low speed and add flour, baking powder, and salt and mix until combined.
Spread mixture into a 9x13 inch-baking pan that has lined with foil, allowing the foil to hang over the sides of the pan by one inch on all sides. Spray with cooking spray.
RHUBARB FILLING
Toss together rhubarb, brown sugar, and flour.
Sprinkle on top of Crumb Bar Base.
CRUMB TOPPING
Stir together melted butter, brown sugar, and salt.
Lightly stir in flour, then ‘smoosh’ mixture together with your fingers so flour forms clumps.
Sprinkle on top of Crumb Bar Base and Rhubarb.
Bake at 350 degrees for about 60-80 minutes, or until internal center temperature reaches 210 degrees.
Cool, cut into bars, and serve.