Preheat the oven to 375 degrees Fahrenheit.
Line a 12 muffin baking pan with muffin liners.
Add the flax meal and water into a small bowl and stir well. Let this mixture rest for 10 minutes, stirring it occasionally to keep it from separating. This mixture is your egg replacement.
In a medium-sized mixing bowl, whisk together the flour, baking powder, and salt. This is your dry mix. Set it aside.
In another bowl, mix the almond milk and vanilla extract. This is your wet mix. Set it aside.
To a standing mixer, with the paddle attachment, add in the cold butter and sugar. Beat at medium speed until light in color, smooth, and fluffy, about 5 minutes. Alternatively, you could use a handheld mixer or beat this by hand with a whisk.
Then, slowly add in the flax egg mixture while the mixer is still beating at medium speed. Scrape down the sides of the mixer's bowl as needed. Once all of the flax mixture is added, continue beating the mix for 30 more seconds to ensure everything is well combined.
Now, turn the mixer's speed down to its lowest setting. Then, gradually add the dry mix and the wet mix into the mixer, alternating between the two. First, add a small amount (about 1/2 cup) of the dry mix into the mixer. Once it is thoroughly combined, add a small amount (about 2 tbsp) of the wet mix. Then add in more of the dry mix, then more of the wet mix, etc. I go back and forth about four times, but starting with and ending with the dry mix.
Mash 1/2 cup of the blueberries slightly with a fork or your hands, and add it into the muffin batter. Add in the other 2 cups of whole blueberries and stir with a rubber spatula until JUST combined.
Spray the muffin liners with your cooking oil spray. Then, divide the muffin batter amongst the muffin pan. Each muffin gets about 1/4 cup of batter.
Sprinkle roughly 1/2 teaspoon of sugar on top of each muffin. Then gently press a few blueberries on top of each muffin.
Place the muffin pan on the middle oven rack and bake it for 30 minutes.
Allow the muffins to cool slightly, then brush them with melted butter and sprinkle on top more sugar, if desired. If you brush butter on top, keep in mind this will soften the tops of the muffins slightly. Alternatively, you can skip the butter or simply spread a piece on top of one before you eat it!