Preheat the oven to 400°F. Coat at 10-inch oven proof 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 out clean, about 25-30 minutes. Serve this warm directly from the skillet, with a big spoon and a bowl of whipped cream to dollop on top.
Rhubarb compote:In a medium saucepan, combine the rhubarb, sugar, lemon zest, lemon juice and arrow root. Bring to a simmer over medium heat, stirring until the rhubarb becomes tender and sauce-like, about 7-10 minutes. Remove from the heat and blend until smooth with a hand-held immersion blender or transfer to a stationary blender. This will keep in the fridge for up to a week.