PREHEAT the oven to 400ºF. COMBINE ½ cup water, ⅓ cup of the sugar, the strawberry jam, and the rhubarb in a 10-inch skillet and bring to a boil over medium heat. Reduce the heat to low, and cook, stirring occasionally, for 3 to 4 minutes, or until the rhubarb is almost tender. Combine the cornstarch and 2 tablespoons cold water in a small cup. Add the mixture to the skillet and cook for 2 minutes, or until the liquid thickens. Turn off the heat and stir in the strawberries. Smooth the fruit into an even layer with a rubber spatula. WHISK the flour, baking powder, salt, and remaining sugar together in a bowl. WHISK the egg, egg yolk, milk, butter, zest, and vanilla together in a large bowl. Whisk in the flour mixture until just combined.
DROP small mounds of the batter onto the fruit mixture, and spread them gently together with a spoon. Bake for 25
to 30 minutes, or until a wooden toothpick inserted into the center of the cake portion comes out clean. Transfer the skillet to a rack and let the cobbler cool for 5 minutes before serving. Rhubarb leaves are toxic, so you’ll probably never see them attached to the stalks. However, if you buy veggies at the farmers’ market, make sure that every bit of leaf is trimmed off before cooking the rhubarb.
Note: The fruit mixture can be prepared up to 2 days in advance and refrigerated in a tightly covered container. Reheat it in the skillet before topping it with the cake batter.