Preheat oven to 350 F and prepare an 9x9 inch pan with non-stick spray and parchment paper both ways for easy removal.
In a mixing bowl, stir together the the gluten free flour, granulated erythritol, baking soda, baking powder, salt and walnuts until combined.
In another bowl, beat together the mashed bananas, eggs, olive oil, vanilla and sour cream until light and fluffy, about 2-3 minutes. Add your dry ingredients to your wet and stir until no dry spots remain. Transfer to the pan and bake for 35-40 minutes or until a toothpick comes out clean. Remove from oven and let cool.
Place the soft coconut oil, honey and cocoa powder into a food processor or blender and blend until smooth. Add in the melted chocolate and continue blending. Frost the brownies and top with pretzels and flaked sea salt. Place in the freezer until chocolate is solid then cut into squares and serve.