The Bojon Gourmet

1 Position a rack in the upper third of the oven and preheat to 350º. Grease a 9x9" square pan, then line it with two crossing pieces of parchment paper, leaving a 1" overhang on each side.
2 In a non-reactive (read: stainless steel, glass or ceramibowl, toss the rhubarb with 1/2 cup of the sugar. Let sit while you prepare the batter and streusel, tossing a few more times. The sugar will dissolve, and the rhubarb will release some juices and soften slightly.
Make the cake batter:
1 In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar and zest on medium speed until light and fluffy, scraping down the sides of the bowl once or twice. Add the yolk, then the egg, and beat to combine. Stir in the vanilla. Sift together the flour, baking powder and salt. With the mixer on low, stir in half of the flour mixture, then all of the sour cream, then the rest of the flour, mixing until just combined after each addition. Remove the bowl and paddle, and give the dough a final mix with a sturdy rubber spatula to make sure it is homogeneous.
Make the streusel:
1 Stir together the flour, brown sugar and salt in a small bowl. Add the melted butter and mix until moist and clumpy.
Bake the buckle:
1 Spread the batter into a thin, even layer in the lined pan. Top with the rhubarb and its juices; spread the slices into an even layer. Break the streusel into hazelnut-sized clumps and scatter them evenly over the rhubarb.
2 Bake the buckle until lightly golden on top, and a toothpick inserted in the center comes out clean, 65 to 85 minutes. Let cool completely, then lift the buckle out of the pan using the parchment handles. Remove the parchment, and cut the buckle into 16 squares.
3 The buckle is best the day it's baked when the streusel is crisp, but it will keep for up to 3 days at room temperature.