In a small bowl, mix the crumb topping ingredients. Use your hands to coat the butter with the flour until it forms small crumbs. If the butter melts and becomes too wet, form a ball and use a spatula to cut it into small pieces, then refrigerate to cool. Once mixed, chop it finely for easier sprinkling later.

In a large bowl, combine milk, oil (use high-quality extra virgin olive oil), egg, and sugar. Weigh the all-purpose flour and baking powder together. Sift the dry ingredients into the liquid mixture and mix until no dry flour remains, but do not overmix to avoid gluten development. The muffin batter should be thick but pourable.

Fill a piping bag or zip-top bag with the muffin batter. Pipe the batter to half the height of the muffin liners, then add blueberries. Pipe more batter until the liners are 3/4 full, add more blueberries, and sprinkle with the crumb topping. Preheat the oven to 350°F (180°C).

Bake in the oven for 25 minutes. Once baked, remove from the oven and let cool before enjoying. To reheat, microwave for 1 minute after refrigerating.