I took about 9 oz (that would be about 250 g, give or take) pitted prunes and pureed them in a food processor. You can also put them in a small saucepan first, add maybe half a cup of water and simmer for about 15 minutes. Then puree. The second method gives it a smoother texture and makes it easier to puree them. But I have done it both ways and both work. Dates would also probably work in this recipe, but I have not tried that.
Add to the prune puree 1 1/4 cups of cocoa powder (A cup in America is 8 fl oz or about 237 ml) and 1/2 cup of flour. I always use whole wheat flour. People tend to use refined flour in baking but whole wheat is so much healthier and in a recipe with this much cocoa, you will not taste the difference. Throw in a cup of sugar. The original recipe has a total of 2 cups of sugar but I like more of a dark chocolate flavor and the prunes do add some sweetness so I find that one cup is enough. If you are a milk chocolate person, you may want to use more sugar.
You may or may not want to add 2 teaspoons of vanilla and a pinch of salt. I sometimes add those, sometimes not. I can't really taste the difference.
That's it. No butter, no eggs, no baking soda. Just prunes, flour, cocoa and sugar. Bake in an 8 x 8 square pan at 300 degrees F for about 45 minutes. They will look a little under baked when they come out, that's okay. Cool completely and refrigerate overnight. If you try to eat them soon after they come out of the oven, they will taste very prune-ish and not too chocolaty. But after being tucked away overnight in the fridge, you will taste only the chocolate and the fudgey texture. They are best eaten at room temperature, but also take good straight out of the fridge.