Preheat oven to 350 degrees Whisk together almond flour, potato starch, sugar, baking powder and guar gum. Add apple sauce, almond milk, melted vegan butter and vanilla extract. Stir until all is incorporated evenly. Batter will be thick. Add in blueberries and mix again. Sprinkle baking soda over batter and add apple cider vinegar over top. Allow to react and become fizzy then fold into batter evenly. Pour batter into parchment paper lined 8x8 pan. TOPPING: Whisk together additional ingredients reserved for the topping--almond flour, potato starch, brown sugar, cinnamon and salt. Cut vegan butter into dry mix. Add pecans and vanilla and work hands though until crumbly. Spread topping evenly over batter and place in oven. Bake at 350 degrees for 60 minutes. At 30 minutes in, rotate the pan 180 degrees and cover with foil. Remove cake from oven and place pan on cooling rack. It will feel a touch soft in the center. It is important to let the cake cool completely before serving. It can then be reheated later if preferred. Cut into squares and drizzle with icing if desired. (To make the icing pictured, just combine 3/4 cup grain-free powdered sugar-I use "Wholesome" brand, 1.5 Tbsp Almond Milk, 1 tsp Vanilla and a dash of salt). Enjoy!