Preheat oven to 425F and line a 12-cavity muffin pan with liners.
To make the streusel topping, whisk together the almond flour, brown sugar, cornstarch and cinnamon. Add the melted butter and mix together until a wet mixture forms. Place in the freezer while you prepare the muffin batter (for about 15-20 minutes).
In a medium-sized bowl, whisk together the almond flour, cornstarch (minus ½ tablespoon), cinnamon, baking powder and salt.
In a large bowl, use an electric mixer on medium-high speed to beat together the eggs and granulated sugar until foamy, about 1 minute.
Add the oil and vanilla and mix until just combined.
Add the blueberries to a small bowl and toss with the reserved ½ tablespoon of cornstarch.
Switch to a spatula and add the dry ingredients to the wet mixture, stirring until just combined.
Gently fold in the blueberries.
Divide the batter into the prepared muffin pan, filling each cavity about ¾ full.
Add the chilled streusel mixture to the tops of each muffin, making sure they are well covered.
Bake at 425F for 5 minutes, then reduce the temperature to 350F and bake for another 13-15 minutes, or until a toothpick inserted into a muffin comes out with just a few crumbs.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Notes
Avoid using large blueberries. I recommend using regular sized or even small blueberries. They disperse more evenly in the batter. Overly large berries can create too much moisture and create soggy spots in the muffins.
Toss blueberries with cornstarch. This will prevent the blueberries from sinking to the bottom of the muffins.
Chill the streusel topping. The melted butter makes the streusel mixture soft and wet. Placing it in the freezer for 15-20 minutes helps firm it up and create crisp clusters on top of the muffins.
Use muffin liners. I like to use liners to prevent these muffins from sticking to the pan and browning too much on the sides and bottom.
Don't overmix the batter. Once you've mixed together the wet ingredients, put the electric mixer aside and switch to a spatula to stir in the dry ingredients and blueberries.