1Preheat oven th. 6 (180 ° C). Break the chocolate into pieces. Melt in a bowl with half-salt butter, 2 min 30 in medium-sized microwave. Mix by turning.
2Mix the sugar then the eggs one by one, mix them gently between each. Add coffee, flour, chopped walnuts.
3Pour into a square silicone mold, smooth the surface and bake for 20 minutes in the oven. Let cool, unmold and cut into portions.