Melt the 1 tbsp of Unsalted Butter in a small bowl in the microwave (~15-30 seconds). Add the 1/4 cup Dark Brown Sugar, 1 tsp Coarse Sea Salt, 2 tbsps of All Purpose Flour, and the 1 tsp of Ground Cinnamon to the melted butter.
Stir together until all dry ingredients have been absorbed by the butter and it now has a coarse sandy texture. Set aside.
To make the Muffins:
Preheat the oven to 375 degrees Fahrenheit. Line a muffin sheet pan with muffin liners and give a quick spray with oil to ensure they do not stick (you can omit the liners and just give it a good spray if you do not have any on hand). Set aside.
In a medium bowl, combine 1 3/4 cup of Blueberries and 2 tbsps of All Purpose Flour. Making sure each blueberry gets fully coated in flour. Set aside.
In the bowl of a stand mixer or a large bowl (if you only have a hand mixer), add the 1/2 cup of Unsalted Butter, 2/3 cup of Granulated Sugar, and 1/3 cup of Dark Brown Sugar. Mix together on medium speed for 2-3 minutes or until the sugars have well incorporated into the butter. Scraping down the sides as necessary.
Add in 1 Egg, Lemon Zest and the 1 tsp of Vanilla Extract. Mix on medium speed for 1 minute. Add in the other 1 Egg and again mix on medium speed until incorporated. Scraping down the sides as necessary.
Add in the 1 1/2 cups of Vanilla Yogurt. Mix on medium speed for 2-3 minutes - making sure to scrap down the sides and the bottom of the bowl so that all gets fully incorporated into the yogurt.
Meanwhile, in a separate large bowl, sift together the 2 1/4 cups of All Purpose Flour, 2 tsps of Kosher Salt*, 1 tbsp of Baking Powder, 1 tbsp of Ground Cinnamon, and the 1/4 tsp of Ground Allspice.
Once the yogurt mixture is fully incorporated, add in half of the flour mixture. Mixing on low speed until incorporated. Scrap down the sides and then add the other half, again mixing on low. The dough should now be formed and it will be quite sticky and thick.
Fold in the blueberries, evenly distributing them throughout the dough.
Scoop the dough into each muffin tin, filling it all the way to the top of the liner and smoothing them out with a butter knife as necessary to create an even top.
Sprinkle streusel across each top (we usually fully coat ours, but the amount per muffin is up to your taste).
Bake for 25-27 minutes until muffins are golden on top or a toothpick comes out clean from the center.
*We use Diamond Crystal Kosher Salt, which is 2 tsps in order to get 2,240mg of sodium. If you are using your store brand or Morton, you may need to use only 1 tsp, but check your salt box to know your sodium level of the salt you have.**Please note that we have provided both volume and weight measurements for the ingredients. We highly recommend using the weight measurements for the best accuracy, but we have provided both just in case you do not own a scale.