In a medium bowl, stir together gluten free flour mix, flax seeds and baking powder. Set aside.
In the bowl of a stand mixer, cream together butter and sugar until smooth and creamy.
Add one egg at a time and beat until well bended.
Add vanilla extract and lemon juice and beat again.
Add the dry ingredients and mix on low speed for 30 seconds. Stop the machine and scrap batter down.
Add half of the almond milk and mix on medium speed for 15 seconds. Add the remaining milk and mix batter on medium speed for 3 minutes.
Set batter aside to rest for 30 minutes.
Meanwhile, spray a 9" baking pan with cooking spray. Line the pan with parchment paper, cut to fit the bottom of the pan. Set aside.
Preheat oven to 350 F.
Toss blueberries with a pinch of flour mix.
Add to the batter and gently fold a couple times with a rubber spatula.
Pour batter into the baking pan and smooth the top.
Bake for 45-47 minutes or until a toothpick comes out clean (no batter sticking to it).
Cool cake in the pan for 15 minutes, then turn out onto a wire rack to finish cooling for 30 minutes. Gently peel parchment off and dust with powdered sugar just before serving.
Video
Notes
Make sure to weigh the ingredients where a weight measurement is given.
Let the batter rest for 30 minutes to allow time for the starches to absorb the liquids.
Tossing the blueberries in flour prevents them from sinking to the bottom of the cake.
An easy way to cut parchment to fit the pan is to trace the pan on the parchment and then cut it out.
Fresh lemon juice was used in the recipe, but bottled juice will work also.
Frozen blueberries can be used, do not thaw before tossing with flour.