Prep: Don't forget to let the sour cream, milk, and eggs come to room temperature for a bit before beginning. Preheat your oven to 350° F.
Make the Streusel: Melt the butter in a medium mixing bowl. Stir in the remaining streusel ingredients. You should have a crumbly mixture. Set aside.
Make the Donut Batter: Melt the butter in a large bowl. Whisk in the sugars, then the eggs. Add the milk, sour cream, vanilla, and lemon zest and whisk until everything is well combined (if there are a few tiny white flecks from the sour cream, that's fine). Add all the dry ingredients, and fold in with a spatula until just a few streaks of flour remain. Add the blueberries at that point, and fold in gently. Don't over mix.
Bake: Spray your donut pan well with nonstick spray. Fill each cavity about 1/2 full. Top each donut with a generous amount of streusel, and gently press it in to the batter. Bake for 9-11 minutes. A toothpick inserted should come out clean or with some moist crumbs, not wet batter. The donuts will also spring back when gently poked.
Serve + Store: Let donuts cool in pan for 5-10 minutes before inverting onto a wire rack. Cool to desired temperature before dusting with powdered sugar and eating! Add any glaze if desired - see below.These are best served fresh from the oven. The streusel will lose it's crunch after the first day, but these can be stored in an airtight container at room temperature for 2-3 days.
Optional Glaze: I made half a batch of streusel and topped the rest of this batch of donuts with a blueberry glaze. To make the glaze, stir together 1 cup powdered sugar, 2 tbsp blueberry preserves, and 1 tsp vanilla extract in a small bowl. Adjust to desired consistency and dunk donuts in the glaze, or drizzle this over top. You can even make this a blueberry lemon glaze by adding a splash of lemon juice. Place donuts on a cooling rack over a baking sheet to catch any glaze drips once you've dunked the donuts.