Preheat the oven to 400F.

In a blender, combine the eggs, milk, flour, and salt; cover and process until smooth.

Place cast-iron skillet over medium heat and stir butter until melted. Continue to cook, slightly simmering, until butter foams, stirring often. Once butter forms toasty brown bits, becomes a golden caramel color, and has a warm nutty aroma, pour in the batter.

Cook for 3-5 minutes and arrange asparagus on the skillet. Bake for 20 minutes, until puffy and sides are golden brown.

Remove from oven and top with prosciutto. Serve immediately.