Begin by zesting both blood oranges and juicing them into a pot. Add 1 1/4 cups sugar, the zest, and split vanilla bean to the pot, bringing to a boil over medium-high heat. Let this simmer for 4-6 minutes to infuse the vanilla flavor and dissolve the sugar completely. Add the trimmed rhubarb pieces and continue cooking for 7-10 minutes, stirring occasionally, until the rhubarb breaks down and the mixture thickens into a jam-like consistency. Transfer to a bowl to cool while you prepare the crust—I like using blood oranges for their subtle tartness, which really complements the rhubarb without being too sharp.

While the jam cools, melt 7/8 cup salted butter in a saucepan over medium heat, stirring occasionally. Continue cooking for about 5 minutes until the milk solids turn golden brown and the butter smells nutty—watch carefully to avoid burning. Pour the browned butter into a bowl and freeze for 35 minutes until it solidifies into chunks. This brown butter gives the shortbread crust a rich, toasted flavor that pairs beautifully with the tart rhubarb filling.

While the butter freezes, combine 1/2 cup powdered sugar and 1 1/2 cups flour in a bowl with a pinch of salt. Once the brown butter is solidified, break it into small chunks and mix into the flour mixture using a fork or your fingertips until the texture resembles coarse breadcrumbs—don't overwork it or the crust will be tough. Press this dough firmly and evenly into a parchment-lined 9x13 inch baking dish, going slightly up the sides. Cover and chill for 30 minutes to prevent the crust from shrinking during baking.

Preheat your oven to 375°F. After the crust has chilled, bake it for 15-18 minutes until it's light golden around the edges but still pale in the center. While the crust bakes, start preparing the custard layer by whisking together 3 room-temperature eggs, 1 1/4 cups sugar, the zest from both blood oranges, and 3/4 cup plus 2 tablespoons flour in a bowl until well combined and pale yellow. Remove the crust from the oven and let it cool for a few minutes while you brown the second batch of butter.

In a separate saucepan, melt 3/8 cup salted butter over medium heat with the remaining 1/2 vanilla bean, cooking for about 5 minutes until deeply browned and fragrant. Remove from heat and let cool to room temperature—this is crucial so the hot butter doesn't scramble the eggs. Once cooled, slowly pour the browned butter into the egg mixture from Step 4, stirring constantly to create a smooth custard. Remove and discard the vanilla bean piece.

Pour half of the custard mixture over the cooled prebaked crust, then dollop most of the cooled rhubarb jam across the surface in spoonfuls—you want swirls, not a solid layer. Pour the remaining custard over the jam, then top with the remaining rhubarb jam and use a thin knife or skewer to swirl through the layers, creating marble-like patterns. I find that less swirling actually looks more elegant than overdoing it. Bake for 25-35 minutes until the custard is just set but still has a slight jiggle in the center—these bars continue to cook as they cool and will firm up beautifully. Cool completely before cutting into squares and serving at room temperature.