In a medium bowl, combine the gluten-free flour, cornstarch, salt, baking soda, and baking powder. Set aside.
In a small bowl, add the granulated sugar and lemon zest and use your fingers to pinch and work the zest into the sugar.
In the bowl of a stand mixer, add the softened butter and lemon sugar and beat it for about 3 minutes, or until light and fluffy. Use a silicone spatula to scrape the sides and bottom of the bowl and the paddle.
Add the eggs one at a time, beating after each addition, followed by the sour cream and vanilla. Scrape the bowl and paddle again.
Add the dry ingredients and beat until smooth. Scrape the bowl and paddle, and turn the mixer on slow while slowly pouring in the milk. Once the muffin batter is smooth, turn the mixer off and scrape the bowl and paddle one last time. Cover the bowl with plastic and let the batter rest for 30 minutes.
In a small bowl, add the streusel ingreidnets and use a fork, pastry cutter, or your fingerstto work the butter into the flour and sugar. Put the streusel in your refregerator or freezer to keep it cold.
Preheat the oven to 410°F (210°C) and line a muffin tin with paper liners. Only add paper liners to every other muffin cup.
When the oven is ready, toss the blueberries in about ½ teaspoon of gluten-free flour and fold them in the batter using a spatula.
Evenly fill the muffin tin with the batter, and top with a sprinkle of streusel.
Bake the muffins for 7 minutes at 410°F (210°C), turn the oven down to 375°F (190°C) without opening the oven door and bake for another 16-18 minutes. Check for doneness by sticking a toothpick into the center of the muffin - it should only have a couple crumbs on it. Let the muffins cool in the pan for 10 minutes before taking them out onto a wire rack to finish cooling.