Preheat the oven to 350F. Grease the bottom and sides of a 9x13 baking dish.
Chop the rhubarb and slice the strawberries. Cover the bottom of the pan with the chopped rhubarb.
In a mixing bowl, coat the strawberries with cornstarch, sugar, and vanilla. Allow to rest for 10 minutes to extract the juices from the strawberries.
While the strawberries are resting, start the cobbler crust by mixing together the flour, sugar, baking powder, and salt. In a separate mixing bowl, combine the butter, milk, egg, and vanilla extract. Slowly pour the wet ingredients into the dry and fold to incorporate until just combined.
After the strawberries have sat, pour the mixture over the rhubarb and spread the crust over the top of the strawberries.