• Preheat the oven to 350 degrees F. Line a muffin pan with paper liners.
• Mix the butter and sugar in a stand mixer fitted with the paddle attachment on medium-high speed until combined and fluffy. As an alternative to a stand mixer, use a large mixing bowl and an electric hand mixer
• Add the eggs one at a time and mix on medium speed. Scrape the sides of the bowl with a rubber spatula as needed.
• Pour in the buttermilk and continue mixing the muffin batter until a smooth batter forms, scraping the sides of the bowl if necessary.
• Stir together the gluten-free flour, baking powder, baking soda, and salt in a separate bowl. Transfer the dry ingredients into the stand mixer with the wet ingredients and beat until a smooth batter forms.
• Fold the fresh blueberries into the batter.
• Fill the muffin holes of the prepared muffin tin ¾ of the way up with batter.

Make the Streusel Topping (optional):

• Combine all of the ingredients for the streusel topping in a small bowl. Sprinkle the topping mixture over the muffin batter. Note: If you aren't making the crumb topping, you can top the muffins with extra blueberries.
• Bake muffins on the center rack of the preheated oven for 22-35 minutes, or until they are golden brown on top and test clean. Insert a thermometer into the middle of a muffin to check its internal temperature for doneness. Muffins are ready once they have reached an internal temperature of 190 to 200 degrees Fahrenheit in the center.
• Remove the muffins from the oven and transfer them to a wire rack to cool completely.