Line 20cm/9” square pan with paper with overhang. Preheat oven 180C/350F (160C fan)
Melt butter and chocolate in a bowl in microwave, 3 - 4 x 30 sec, stir in between. Stir through sugar (brings temp down), then egg, then remaining ingredients. Quickly stir chopped choc in last, if using.
Pour into pan. Bake 24 min for really gooey, 28 min for fudgey (my fave, per video!) and 32 min for less fudgey but still crazy moist, like a fudge cake.
Cool in pan, lift out with overhang paper, cut into 9 or 16 squares. Start your diet
