To make the prune puree, put the prunes and water in a small saucepan. Cover and bring to a boil, then reduce the heat so it’s at a simmer and cook for 5 minutes. Turn off the heat and let the prunes rest, covered, until they’re room temperature. Once cool, puree the prunes in a mini-chopper or food processor, so they’re as smooth as possible, or pass them through a food mill. Measure out ½ cup of the puree.

Preheat the oven to 325ºF (160ºC). Line one or two baking sheets with parchment paper or silicone baking mats.

Put the chocolate in a medium clean, dry bowl. Set it over a saucepan of barely simmering water. Stir it gently, until it’s completely melted. Turn off the heat and set aside.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In the bowl of a stand mixer fitted with the paddle attachment (or by hand), beat the butter and brown sugar together on medium-high speed until smooth and well-combined, about 2 minutes.

Stop the mixer, add the prune puree (or eggs) and beat on medium speed until the puree is incorporated.

Stop the mixer and add one-third of the dry ingredients, then mix in on low speed. Add the second third of the dry ingredients, stopping the mixer and scraping down the sides of the bowl as well as from the bottom, to make sure any dry ingredients aren’t sinking to the bottom and not getting mixed in. Finally, mix in the remaining dry ingredients.

Remove the bowl of chocolate from the saucepan, wipe the bottom dry, then mix it into the dough. Stir in the chocolate chips.

Use a spring-loaded ice cream scoop (one that holds around 2 tablespoons/45g) or two soupspoons to scoop up rounds or mounds of the dough.

Place rounds of dough 2 ½ inches (6cm) apart, evenly spaced, on a baking sheet or sheets (you’ll likely only get 6 to 8 per baking sheet, so you’ll have to bake them in batches) and bake the cookies, rotating the baking sheet(s) in the oven midway during baking until the cookies are puffed up and feel just barely cooked in the center, but still quite soft, about 9 minutes.

Err on the side of underbaking rather than overbaking. Remove the cookies from the oven and rap the top of each cookie once with a spatula or pancake turner to flatten them a bit, which compresses the cookies and makes them chewier.

Eat the cookies warm or at room temperature. The cookies are best eaten the same day but will keep a few days in an airtight container at room temperature.