Preheat the Oven: Set your oven to 400°F and prepare a 12-cup muffin tin by lining it with paper liners. Set the tin aside for later.
Make the Crumb Topping: In a small mixing bowl, combine the flour, sugar, sea salt, and cinnamon. Add the softened butter and use your fingers to mix the ingredients together until it forms coarse crumbs. Once ready, set the mixture aside.
Combine Dry Ingredients: In a medium-sized bowl, whisk the flour, baking powder, salt, and cinnamon together until well mixed.
Cream Butter and Sugar: In a stand mixer fitted with the whisk attachment, or using a handheld mixer in a large bowl, beat the butter and sugar on high speed for about 2 minutes until the mixture becomes fluffy and pale. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
Mix Batter: Add half of the dry ingredient mixture into the butter and sugar mixture, followed by ¼ cup of milk. Mix on medium speed until just combined. Repeat with the remaining dry ingredients and the rest of the milk. Be careful not to overmix; the batter will be thick.
Fold in Blueberries: Using a spatula, gently fold the blueberries into the batter, making sure they are evenly distributed.
Fill Muffin Tin: Divide the batter evenly among the prepared muffin cups, filling them nearly to the top. Though the cups will look quite full, this is fine.
Add Topping: Sprinkle about 1 tablespoon of the prepared crumb topping over each muffin, concentrating it towards the center so it doesn’t spill off during baking.
Bake the Muffins: Place the muffins in the oven and bake for approximately 25 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the muffins to cool in the tin for about 10 minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.