Preheat oven to 375°F. Line a square 8x8 or 9x9 baking dish with parchment paper and spray with cooking oil. Set aside. In a large bowl, whisk together oat fiber, cocoa powder, sweetener, instant espresso, baking powder and salt. Set aside.
In a medium bowl, pour in egg whites. Using an electric mixer, mix on medium speed until frothy. Then increase speed to high and continue to beat until stiff peaks form. Set aside.
To the bowl with the dry ingredients, add yogurt, milk and vanilla. Mix until combined.
Add a third of the egg meringue mixture to the chocolate batter and fold in carefully trying to keep as much air as possible in the batter. Add another third of the egg mixture and fold in. Once incorporated, gently fold in the remaining egg mixture.
Pour the brownie batter into the prepared pan and bake at 375°F for 25 minutes or until the brownies begin to crack around the edges and is puffy on top. The batter may still be a little wet in the middle but will solidify as it cools. Remove from the oven and let cool completely.
Using a sharp, clean knife, cut the brownies into 16 squares to make them 20 calories each.
