Whisk the eggs, milk, vanilla extract, and cinnamon in a shallow bowl until smooth.
Dip each slice of brioche bread into the egg mixture, coating both sides, and set aside on a plate.
Heat 1 tablespoon of butter in a large skillet over medium heat. Cook the dipped bread slices in batches until golden on both sides, adding more butter as needed.
While the French toast cooks, combine the mixed berries, granulated sugar, and lemon juice in a small saucepan. Simmer over medium heat for 5 to 7 minutes, stirring occasionally, until the berries soften and release juice.
Cut each cooked French toast slice into large triangles. Stack three triangles per plate to form a witch hat shape.
Spoon the warm berry sauce over and around each stack. Dust lightly with powdered sugar before serving.