Put a rack in the upper third of the oven and preheat the oven to 375° F. Line a 12-cup standard muffin pan with paper or foil liners. For the muffins, in a medium bowl, whisk together the milk, egg, egg yolk, maple syrup, and vanilla. Add the butter and stir well.
In a separate bowl, whisk together the flour, sugar, baking powder, and salt. Add the milk mixture and stir gently. Fold in the blueberries. Divide the batter among the prepared muffin cups. Bake until golden and crisp on top, and a wooden pick inserted into the center of a muffin comes out clean, 18 to 20 minutes. .
For the maple glaze, in a small saucepan set over low heat, combine the maple syrup and lemon juice. Bring to a simmer and cook, stirring occasionally, until reduced to a thick (thicker than maple syrup) syrup, about 15 minutes. Remove the pan from the heat.
When the muffins are out of the oven and cool enough to handle, poke a few holes in each muffin top with a toothpick and drizzle each muffin top with the maple glaze. Muffins will keep in an airtight container at room temperature for up to three days