Whisk milk, eggs, maple syrup, vanilla, and cinnamon together in a medium bowl. Add crumbled cornflakes and stir to combine.

Working in batches, soak 1/2 of the bread slices in the mixture for about 2 to 3 minutes.

Heat oil in a large skillet or griddle over medium-high heat. Cook soaked bread slices in the hot skillet until dark golden brown, 4 to 5 minutes per side. Repeat to soak and cook remaining bread.

Sprinkle hot French toast with confectioners' sugar. Serve immediately.