Preheat oven to 425°F and set a 10-inch oven-safe skillet inside to heat.
Whisk together the milk and eggs. Add the flour, salt, herbs, and a few cracks of black pepper. Whisk until very smooth and no lumps of flour remain.
1/2 cup milk, 3 large eggs, 1/2 cup all-purpose flour, 1/4 teaspoon salt, Cracked black pepper, 2 tablespoons minced fresh herbs
If you want to use a blender to make the batter, add the herbs after blending.
Remove the hot pan from the oven and add the butter. It will immediately sizzle and start melting. Carefully tilt the pan to coat the butter all over, including up the sides of the pan.
4 tablespoons unsalted butter
Pour in the batter and return the pan to the oven. Bake for 15-20 minutes, until puffed and golden.
While it is baking, toss the arugula with the olive oil and balsamic, and a pinch of salt and pepper.