For the Muffins:

1. Preheat the oven to 375°F and line a 12-cup muffin pan with paper liners (this recipe makes about 16 muffins for me, so I prepare another 6-cup muffin tin as well).
2. In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon.
3. Using a hand or stand mixer, beat the butter and sugar together until pale yellow and fluffy, about 2 minutes (you can also just whisk together by hand).
4. Add the eggs, vanilla extract, and oil and beat until smooth.
5. Alternate adding the flour and the milk, starting and ending with flour (so, add half the flour first, then all of the milk, and end with the remaining half of flour), gently stirring and folding the batter after each addition. Try not to overmix! This will yield tough muffins.
6. To the empty flour bowl, add the blueberries and chocolate chips and 1 tablespoon flour. Gently toss to evenly coat in the flour.
7. Using a spatula, gently fold the berries and chocolate chips into the batter.
8. Scoop the batter into the prepared muffin pan and then top each muffin with the streusel topping.
9. Bake for 20-25 minutes or until the muffins are golden around the edges and set in the middles.
10. Allow to cool on a wire rack and keep covered at room temperature in an airtight container for up to one week.

For the Streusel Topping:

1. To the same bowl used for the muffin dry ingredients, whisk together the flour, sugar, cinnamon, and a tiny pinch of salt.
2. Add the melted butter and stir the mixture until it clumps together and is crumbly.
3. Generously top each muffins with the streusel before baking.