For rhubarb, stir rhubarb, sugar, cornstarch and scraped seeds from vanilla bean and/or lavender in a small saucepot. Slowly bring up to a simmer, stirring only occasionally, until rhubarb is tender and sauce is shiny. Cool, then chill completely.
For cupcakes, preheat oven to 350 °F and line a 12-cup muffin tin with paper liners. Sift flour, sugar, baking powder and salt. Add butter, sour cream, egg, egg yolks and vanilla and mix on low speed until blended. Increase speed to medium-high for 15 seconds, then spoon batter into prepared muffin cups. Bake for 18 to 22 minutes, until a tester inserted in the center of a middle cupcake comes out clean. Cool completely in the tin.
To fill cupcakes, take a paring knife and carve out a whole in the center of each cupcake. Spoon a bit of the rhubarb into each cupcake and place back in the muffin tin.
For meringue, preheat oven to 425 °F. Whip egg whites with cream of tartar until foamy. Gradually add sugar while whipping and continue until whites hold a stiff peak when beaters are lifted. Dollop or pipe meringue on rhubarb filling of each cupcake and bake for 3 to 4 minutes until lightly browned, or use a butane torch. Allow to cool before serving.
Cupcakes can be prepared up to a day in advance and stored chilled. Serve cupcakes with remaining compote on the side.