Preheat the oven to 375 degrees. In a large mixing bow, whisk the egg yolk, egg, milk, maple syrup, vanilla, and melted butter together until smooth. Add the flours, baking powder, and salt and stir until just combined.
Rinse the blueberries under cold water until the water runs mostly clear. Gently stir the blueberries into the batter. Scoop into a prepared muffin tin (Spray the pan or use paper liners. Use an old-fashioned ice cream scooper to scoop the batter in perfectly!). Transfer the muffin tin to the oven.
Bake for 20 minutes until the tops are springy when you touch them. Remove from the oven and allow to cool.
To make the glaze, melt the butter in a small saucepan. Add the milk and stir until combined. Add the powdered sugar and whisk until you've gotten rid of all the clumps. If it's too thin, add another ¼ cup powdered sugar. The glaze should be smooth and glossy, but as soon as you remove it from the heat, it will start to harden. Drizzle the glaze over the muffins - it should set almost immediately.