MAKE THE COMPOTE in a medium heavy-bottomed saucepan, combine the rhubarb, sugar, lemon zest, lemon juice, and cornstarch. Bring to a simmer over medium heat, stirring constantly until the rhubarb becomes tender and sauce-like, about 5 minutes. Remove from the heat and allow to cool to room temperature. This will keep in the fridge for up to a week.
MAKE THE CAKE preheat the oven to 400°F, coat a 10-inch ovenproof skillet, preferably cast iron, with butter and flour, shaking out any excess flour.
In a medium bowl, combine the flour, baking powder, sugar, and salt. In a large bowl, whisk together the egg, vanilla, milk, sour cream, and melted butter. Gently stir the wet ingredients into the dry until just incorporated.
Pour about two-thirds of the compote into the greased skillet and spread evenly. Pour the cake batter over it, spread evenly, then dollop the remaining compote over the top. Use a butter knife to swirl together the batter and compote.
Bake until a cake tester or knife inserted in the middle of the cake comes our clean, about 25 minutes. I like to serve this warm, directly from the skillet, with a big spoon and a bowl of whipped cream to dollop on top.
