Whisk flour, sugar, and salt together in large bowl. Whisk milk and eggs together in second bowl.
Whisk two-thirds of milk mixture into flour mixture until no lumps remain, then slowly whisk in remaining milk mixture until smooth.
Adjust oven rack to lower-middle position. Melt butter in 12-inch ovensafe nonstick skillet over medium-low heat. Add batter to skillet, immediately transfer to oven, and set oven to 375 degrees.
Bake until edges are deep golden brown and center is beginning to brown, 30 to 35 minutes. Gently transfer Dutch baby to cutting board. Let cool for at least 5 minutes. (Dutch baby will deflate.)