Preheat the oven to 375 F (190 C) and grease, then line a loaf pan with baking paper, then grease the paper.
Using an electric mixer, cream together the butter and the sugar, then mix in the eggs and vanilla.
Using a spatula, gently fold in the melted (and cooled) chocolate.
Stir the baking soda into the flour, then use your spatula to gently fold the flour mixture in, alternating every little bit with a spoonful of the boiling water, until the batter is smooth and liquid.
Pour the batter into your prepared loaf pan and bake for 30 minutes, then turn the oven down to 325 F (160 C) and bake for about 20 minutes more. (the cake should not appear to be cooked through, a toothpick test should not come out clean, but when you tilt the pan you should not see much swelling, if you do, cook for an additional 5 minutes)
Put the pan to cool on a wire rack, and let it cool completely. (most likely the cake will fall in the middle, as loaf cakes tend too, but no worries, it is just space to fill with fruit sauce or vanilla ice cream ; )