Preheat oven to 375F and grease your muffin tins. Adding the blueberries, lemons and vanilla gives you 15 regular sized muffins when the wells are filled just to the rim. I used butter instead of paper liners because the liners always sticks to the muffins. The butter also helps the muffins to brown. If you're using oil instead of butter, use the grapeseed oil, applying sparingly with a pastry brush.
Zest the two lemons
Using your hands, or a the citrus squeezer, juice both lemons. Get as much juice as you can and then scrape some of the pulp out of the lemons as well.
Beat the 3 eggs. Add the milk, oil and vanilla to the eggs, lemon juice and zest. Mix well.
Dump the box ingredients in a large bowl. Add all the wet ingredients. Mix just until blended. Some lumps are ok. You do not want to over mix muffin batter.
Fold in the blueberries just until blended. Do not stir.
Grease your muffin tin and then fill each well with 4 Tbs of batter. This will fill them to just below the rim. Let them sit for 10 minutes before putting them in the oven.
Bake for 18 to 22 minutes. The time will depend on your oven and how much you have filled the muffin wells. They are done when a toothpick or slim sharp knife inserted in the center of the muffins comes out clean. Let cool in the tin for 5 minutes, giving time for them to set. Once you take the muffins out, resist the urge to eat them right away. Warm Gluten Free bakery can be gluey or glumpy. Letting it cool off, will bring the flavors out and lighten up the texture.