Add unsoftened butter, brown sugar, and all purpose flour to a small bowl. Mix with your hands until crumbly. (See pictures above for "how to make large pea-size crumbs".) Cover the bowl with lid or food wrap, keep it in the refrigerator.
Wash and dry blueberries. You can pat the blueberries dry with paper towel, or let them air dry on a wire rack. Do not thaw the blueberries if you are going to use frozen ones.
Preheat oven to 350° F.
Add a stick of unsalted butter to a small bowl, microwave for 30 seconds, stir, then microwave for 15 seconds until it's fully melted.
Crack 2 eggs to a small bowl, whisk until well combined. Weigh 80g egg liquid to a medium bowl, add milk, vanilla extract, and melted butter to the bowl, whisk until well combined.
Add all purpose flour, brown sugar, and baking powder to a large bowl, whisk until well combined.
Pour the egg milk mixture to the large bowl, fold and stir with a spatula until there is no dry flour in the bowl. Do not over mix.
Line a muffin pan with 12 muffin liners (paper or silicone). Scoop and evenly divide ⅓ of the batter to 6 of the muffin cups* (as shown in pictures above). Add 4 to 5 blueberries to each cup, gently press the blueberries into the batter. Scoop and evenly divide another ⅓ of the batter to the 6 muffin cups. Add 4 to 5 blueberries to each cup, gently press the blueberries into the batter.
Scoop and evenly divide the rest of the ⅓ of the batter to the 6 muffin cups. Make sure you use up all the batter. Decorate 3 to 4 blueberries on the top surface of the batter.
Sprinkle the crumb topping over the surface of the batter.
Use a brush to sweep the crumb topping scattered on the muffin pan into the empty muffin liners. Remove the empty muffin liners.
Place the muffin pan on the upper third rack of the oven. Bake at 350° F for 30 minutes.
30 minutes later, increase the oven temperature to 400° F, bake for 3 to 5 minutes until the crumb topping turns golden. Observe carefully at this step to prevent burning.