Preheat your oven to 170°C fan / 375°F / Gas 5 and line a 2-pound loaf pan (900g) with baking or parchment paper.
In a medium saucepan, melt together the butter, sugar and black treacle, stirring until the sugar has dissolved. Remove from the heat and stir in the milk. Set aside to cool until lukewarm, then beat in the eggs.
Sift the flour, cocoa, bicarbonate of soda, ground ginger and salt into the saucepan and stir until you have a smooth batter. Finally, incorporate the crystalized ginger (if using).
Pour the batter into the prepared pan. Give it a sharp tap on a work surface to get rid of any air bubbles.
Baking: Place in the pre-heated oven and bake for 45-50 minutes until a toothpick or skewer comes out with a sticky crumb or two attached. Watch it carefully as it has a tendency to easily over bake. Tent the cake with foil for the final 15 minutes if it is browning too quickly.
Allow the cake to cool in the baking pan for 15 minutes, then transfer to a wire rack to cool completely.
Buttercream: When the cake has cooled. Mix all the ingredients together in a bowl and then pour it over the cake and spread it evenly using a spatula. Decorate with pieces of crystalised sugar.
Chocolate drizzle: Finally, melt the chocolate in a small bowl over a simmering saucepan of water or in the microwave. Drizzle over the top from side to side and then leave to set before slicing.