Melt 2 tablespoons butter in a cast iron skillet
Add melted butter to a food processor, then add milk, flour, eggs, sugar, vanilla, and lemon zest
Blend until smooth
Melt remaining 2 tablespoons butter in the skillet
Add blueberries and cook for 30 seconds
Pour egg mixture over the blueberries
Bake at 450°F until puffy and golden brown, about 15 minutes
