Preheat oven to 180 degrees. Line a 9x13 inch pan with foil or parchment paper. Butter it generously. Set aside.
Melt the butter and chocolate till smooth. Whisk in the coffee powder if using. Set aside to cool slightly.
Beat the eggs, sugar and vanilla extract till thick and doubled in volume.
Add the melted butter-chocolate and beat till combined.
Sift in the flour, cocoa powder and salt and fold in. Fold in the chocolate chips.
Pour into the prepared pan and bake for 30 minutes.
Pull out of the oven and cool in pan completely. Best is to cool it overnight.
Slice and enjoy!