Adapted from Honestly Yum
Preheat oven to 425°F. Prepare a muffin tin with liners and set aside.
First, make the streusel crumble: Combine the two sugars, salt, and flour in a small bowl, and then slowly whisk the melted butt into the mixture with a form. Set aside.
Then, make the blueberry glaze: combine 1 cup blueberries with 1 teaspoon granulated sugar in a small saucepan over medium high heat. Mash the blueberries with a potato masher (or the back of a spoon) and bring the mixture to a simmer. Simmer, stirring frequently, for about 5 minutes until combined and broken down, then remove from heat. Be careful not to overcook.
Finally, the muffins: In your biggest bowl, whisk together the flour, baking powder, and salt. In a separate bowl, whisk together the eggs and sugar, and then the browned butter and oil. Mix in the milk. Fold the egg mixture into the dry ingredients, adding the remaining 1 cup blueberries and fresh lemon zest as you do so. Mix until just combined.
Fill the each muffin cup about 2/3’s full with batter. Spoon 1 teaspoon of blueberry glaze on top of each, and swirl it around to cover the top of the batter with a toothpick (or fork). Sprinkle the streusel on top of each muffin. Bake for 20-25 minutes, remove from oven, and let cool.
Why Browned Butter? Certainly, this recipe would work just fine with regular melted butter, but don't be intimidated if you've never used brown butter before. It can add a lovely, rich layer of flavor to baked goods.
Melt the butter in a small sauce pan over medium heat. It will start to turn a deep golden yellow then brown (like a nice whiskey) and foam. As soon as the foaming bubbles slow and stop, remove the pan from heat and transfer to a heat-proof bowl. It should have a nutty aroma. You can either strain out the solids, which will have become a sediment at the bottom of the pan, or leave them in them. I don't mind the slight burned flavor they add and usually leave them in.