Mary Berry's 1 Unexpected Secret Ingredient For The Best Chilli

It’s amazing how much difference a single ingredient can make, isn’t it? 

Baking powder can transform overly tart tomato sauces into perfectly balanced delicacies. Sundried tomatoes, Mary Berry tells us, can make an otherwise regular soup sing. 

And as all too many of us have heard, a square of dark chocolate can add a delicious depth to chilli. 

Those are all valid points, of course. But Mary Berry swears by another, more unorthodox addition to make her chilli recipes sing. 

Which is?