Stir the water and chia seed together in a small bowl. Cover and let rest overnight.
The next day, melt chocolate in a saucepan over low heat. Remove from heat and set aside to cool. Meanwhile, preheat oven to 350’ F. Line a baking pan with foil, leaving enough at the ends to fold over. Lightly spray with cooking spray. Set aside.
Whisk sugar into cooled chocolate. Add eggs, one at a time, and whisk just until blended. Add chia seed mixture and vanilla; stir to combine.
Combine flour, baking soda and espresso coffee powder in a small bowl. Add this mixture to the chocolate mixture; stir to blend. Pour batter into prepared pan, spreading evenly with a spoon or spatula. Bake 30 minutes. Let cool before cutting into squares.