Chocolate Loaf Cake

INGREDIENTS

• zest of one lemon
• 1 1/4 cups / 5 oz / 145 g spelt flour
• 1 teaspoon baking soda
• 1/2 teaspoon fine grain sea salt
• 1 cup / 8 oz / 225g unsalted butter
• 1 2/3 cups / 7.5 oz / 215 g v. loosely packed dark Muscovado sugar
• 2 large eggs, beaten
• 2 teaspoons vanilla extract
• 4 oz / 115 g bittersweet chocolate, barely melted
• 1 cup plus 2 tablespoons boiling water
• another 1/3 cup / 1.5 oz / 45 g Muscovado sugar (optional)