Make the date puree: In a medium saucepan, boil the water and the dates until the dates begin to soften and almost fall apart. Stir in the baking soda, then transfer the mix to a food processor or blender. Blend until it begins to turn into a paste/puree (it doesn’t need to be perfectly smooth for these cookies).
Whisk together the flour, baking soda, salt, instant espresso, and cocoa powder; set aside. In the bowl of a stand mixer, or with a hand-mixer, cream together the butter, brown sugar, and date puree until light and fluffy.
Add the egg, vanilla, and molasses one at a time, mixing until combined between each addition.
Half at a time, add the dry ingredients. Mix until the flour is just incorporated and the dough begins to come together. Fold in the chopped dates and walnuts. Allow the dough to chill in the refrigerator for at least 30 minutes.
Preheat the oven to 325°F and line a baking sheet with parchment paper. Scoop the dough onto the prepared sheet and bake for 12-15 minutes. Immediately transfer the cookies to a cooling rack.
Make the Toffee Sauce: Put all of the ingredients for the toffee sauce into a medium saucepan. Whisk the ingredients together until the butter has melted and the sugar has dissolved. This should only take about 5 minutes or so. Allow the sauce to cool and thicken a bit before pouring on top of the cooled cookies.