Preheat the oven to 375 degrees F. Grease a 3-quart casserole dish with 2 teaspoons of butter and set aside.

Bring a pot of salted water to a boil. Add the cauliflower and cook until tender but still firm, 8 to 10 minutes. Transfer with a slotted spoon to a colander, rinse under cold running water, and drain well.

Return the water to a boil, add the broccoli and cook until tender but still firm, about 5 minutes. Drain in a colander, refresh under cold running water, and drain again.

Place the cauliflower in the prepared pan and top with the broccoli.

In a medium heavy saucepan, melt the remaining stick of butter over medium heat. Stir in the flour, whisking constantly over medium heat until thickened into a blond roux, about 4 minutes. Add the remaining 2 teaspoons salt and the cayenne, stir, and gradually add the cold milk, whisking constantly until the mixture thickens, about 4 minutes. Bring to a simmer and add the cheese, whisking constantly until the cheese is melted. Remove from the heat.

Pour the cheese sauce over the vegetables, gently rapping the casserole dish on the counter top to dispel air bubbles.

In a bowl, mix together the bread crumbs, oil, and Essence and evenly distribute over the top of the vegetables. Bake until golden brown, about 30 minutes. Serve hot.