Rhubarb Cake Squares

This dessert belongs in everyone's "outdoor entertaining" file. The cake squares, embellished with moist fruit and a crunchy topping, are meant to be eaten out of hand.

Rhubarb Cake:
vegetable oil cooking spray, for baking pans
1 lb + 10 oz rhubarb, trimmed and cut ½" thick on the bias
2 cups sugar, divided
2 cups all-purpose flour, unbleached
1¼ tsp. baking powder
1 tsp. coarse sea salt
1½ sticks butter, softened
1 tsp. lemon zest, finely grated
3 large eggs
1 tsp. pure vanilla extract
½ cup sour cream

Topping:
1 cup all-purpose flour, unbleached
¼ cup light brown sugar
¼ tsp. coarse sea salt
½ stick butter, melted

Preheat oven to 350°F (180°C/160°C fan, Gas Mark 4), with rack in center position. Coat two 9" square cake pans with cooking spray and line with parchment paper, leaving an overhang on both sides.

Make the Cake: 
Stir together rhubarb and 1 cup sugar; set aside to macerate.  Whisk together flour, baking powder and salt. Beat together butter, remaining cup sugar, and the lemon zest until light and fluffy. Beat in eggs, 1 at a time, then beat in vanilla. Beat in flour mixture in 2 additions, alternating with sour cream, beginning and ending with the flour mixture.

Make the Crumb Topping:
Stir together the flour, brown sugar and salt. Add melted butter; stir to combine. Divide batter between pans. Top with the rhubarb mixture and sprinkle with the crumb topping.

Bake until golden on top and cooked through, about 1 hour 5 minutes. Let cool completely in pans on wire racks, then lift from pans using parchment. Remove parchment.

Before serving, cut into 2" squares.

Makes 32 squares.