Preheat hot: Heat the oven to 425°F. Line a standard 12-cup muffin tin with paper liners. The high starting heat is what gives these muffins their tall domes, so get the oven fully hot before the batter goes in.
Whisk the dry: In a large mixing bowl, whisk together the flour, baking powder, cinnamon, and salt. Stir in the brown sugar and break up any lumps with your fingers so the sugar is evenly distributed.
Combine the wet: In a large measuring cup or small bowl, whisk the milk and beaten eggs, then pour in the melted (but not hot) butter and whisk once more. Hot butter would scramble the eggs, so let it cool for a few minutes after melting.
Fold with a light hand: Pour the wet mixture into the dry and fold with a rubber spatula for just 10 to 12 strokes. Stop as soon as no dry flour is visible — the batter should look lumpy and shaggy.
Rest the batter: Let the bowl sit at room temperature for 20 minutes (optional but worth it). The flour hydrates, the batter thickens slightly, and the muffins bake up noticeably taller.
Fill generously: Gently fold in the blueberries with two or three strokes. Divide the batter among the 12 cups, filling each nearly to the top. Sprinkle with coarse sugar if using.
Bake hot, then low: Bake at 425°F for 5 minutes, then — without opening the oven door — reduce to 375°F and bake 15 to 18 minutes more, until deeply golden and a toothpick comes out clean or with moist crumbs.
Cool briefly: Rest in the tin for 5 minutes, then lift onto a wire rack. Serve warm or cool completely before storing.
