Adapted from Alexandra Cooks (via Martha Stewart and Rosebank Farms Café via Gourmet Magazine, July 2004)
Yield = 16 squares
Notes: I always have leftover dough and crumb topping when I make this recipe. In my opinion, you can cut back on the crumb topping a bit or you can use the leftovers for another delicious fruit dessert. If you are using a different fruit, you need about 1 pound (after removing any pits, etc.) or approximately 4 cups. The quantity of fruit does not have to be exact.
9 ounces rhubarb, trimmed and cut 1/2 inch thick on the bias (approximately 2 1/2 cups)
7 ounces strawberries, tops removed and thinly sliced (approximately 1 1/2 cups)
1/8 cup sugar (The original recipe, which is a straight rhubarb filling, called for 1/2 cup of sugar. I've decreased this considerably since I added strawberries and it was plenty sweet. For other fruit, use your judgment. I find that most fruit is sweet enough that you need to add very little sugar. Rhubarb is at the bitter end of the sweetness spectrum, so I'd never use more than 1/2 cup of sugar with any fruit.)
1 teaspoon finely grated lemon zest
1 1/2 cups all-purpose flour
1/4 cup sugar
1 stick cold, unsalted butter
2 egg yolks
2 tablespoons heavy cream
1 cup all-purpose flour
1/4 cup light-brown sugar
1 teaspoon salt
6 tablespoons unsalted butter, softened
1. Make the crust: Whisk together flour and sugar in a large bowl. Blend in butter with your fingertips or a pastry blender until mixture resembles coarse meal with some roughly pea-sized butter lumps. Beat together yolks and cream with a fork and stir into flour mixture until combined. Gently knead mixture in bowl with floured hands just until a dough forms. These steps can also be completed using a stand mixer. Flatten dough into a 6-inch disk and chill, wrapped tightly in plastic wrap, at least 1 hour.
2. Preheat oven to 400 degrees, with rack in center position. Line a 9-inch square cake pan with parchment paper.
3. Stir together rhubarb, strawberries, 1/8 cup sugar and lemon zest; set aside to macerate. If you don't do this well in advance and all the sugar isn't soaked up, don't worry about it - just scrape it over the fruit and it will soak in during baking.
4. Crumb topping: Stir together flour, brown sugar, and salt. Add the butter and mix up with your fingers until clumps form. Set aside.
5. Unwrap dough. The recipe yields enough for a 9x13 pan so you will have some leftover. Roll out the dough between 2 sheets of parchment paper (or wax paper) into a square, or as close to this shape as possible. Peel off top layer of parchment and invert dough into prepared baking pan. This transfer won't be perfect and there will be some wonkiness/folds in the corners. Trim the dough at the edges and patch the corner holes with trimmed dough. If you have trouble peeling the dough from the parchment, put the dough back in the refrigerator until it is chilled enough that the dough separates easily - it shouldn't just fall off, but this shouldn't be hard to do and warm dough will make it hard.
6. Top this crust layer with rhubarb mixture, and sprinkle with as much crumb topping as you would like. Bake for 30 minutes, then reduce heat to 350 and bake for about 35 minutes more or until golden on top and cooked through. Let cool completely in pan on wire rack, then lift cake from pan using parchment. Remove parchment. Before serving, cut buckle into 2-inch squares. Note that the longer this sets before eating, the less runny it will be and the better your slices will hold together in squares.