Blueberry Muffin Tops

INGREDIENTS

• 3 tablespoons cold unsalted butter, cut into bits
• 2 cups fresh blueberries (12 oz)
• 3/4 stick (6 tablespoons) unsalted butter
• 1 1/2 teaspoons baking powder
• 1/2 cup all-purpose flour
• 3/4 teaspoon salt
• 3/4 teaspoon vanilla
• 3/4 cup sugar
• 1/3 cup whole milk
• 1 1/2 cups all-purpose flour
• 1 whole large egg
• 3 1/2 tablespoons sugar
• 1 large yolk