In the mixing bowl of an electric mixer, cream butter and sugar on medium high speed for 5 minutes.
Reduce speed to low and add eggs, lemon juice, zest and vanilla.
In a separate bowl, whisk together gluten free flour, baking powder, baking soda and salt.
Slowly add dry ingredients to mixer set to low speed.
When dry ingredients are incorporated, stir in sour cream with a wooden spoon or spatula.
Fold in blueberries, then scrape batter into buttered 8 x 8pan.
In another bowl, whisk together gluten free flour, brown sugar, sugar and cinnamon. Pour in melted butter and stir with a fork until crumbs form. It’s ok to leave some larger crumb pieces with the smaller ones.
Sprinkle crumb mixture evenly over the top.
Bake at 350° for 45 to 55 minutes, until a tooth pick inserted in the center comes out clean.
Remove from oven and cool on wire rack for at least 30minutes.