Heat oven to 375°F.
Line a 12 cup muffin tin with muffin papers.
Stir the butter and sugar together until combined.
Add egg and vanilla, and mix well.
Mix flour, baking soda and salt together in a small bowl.
Add half the flour alternatively with half the milk, stir until just combined.
Repeat with remaining flour and milk.
Add ½ cup of blueberries and stir to break up the blueberries into the batter.
Gently fold the remaining berries into the batter.
Fill prepared muffin tin, filling each cup ⅔ full.
Bake in preheated oven for 25-30 minutes.
Cool in pan 2-3 minutes before removing from pan to cooling rack.
Store in airtight container for up to 3 days.