Preheat the oven to 325° and line a 11x7 pan with tin foil and spray with non-stick spray.
In a small saucepan over medium heat, melt butter and chocolate together, stirring constantly, until completely smooth. Set aside.
In a small bowl, mix the all purpose flour, cake flour, and salt together. Set aside.
In a large bowl, beat the sugar and eggs until light and fluffy. I did this with my hand, vigorously stirring for a couple minutes. You can also use an electric mixer.
Add the chocolate mixture, vanilla, and coffee- stir to combine.
Gently add in the flour, mixing until just combined.
Spread batter into prepared pan and bake for 25-30 min or until a toothpick inserted into the center comes with just a few moist crumbs. Do not over bake them.
Let cool completely in the pan, then remove them using the foil and cut into squares.