Preheat the oven to 190°C (370°F) or 170°C (340°F) for fan-forced ovens. Grease a 30x11x7cm (12x4x3in) loaf tin with melted butter, then dust with flour. Tap the tin upside down to remove the excess flour.
For the egg mixture - In a bowl, lightly whisk together the egg, milk, liqueur, orange juice, orange zest and vanilla extract just until combined.
For the flour mixture - Sift the flour, salt and baking powder into a stand mixer bowl, then add in the sugar. Whisk well to combine.
Place HALF of the egg mixture and all of the butter into the flour mixture. Using the paddle attachment, beat the ingredients on low speed until combined, then increase the speed to medium-high and beat for 1 minute. Scrape down the sides and bottom of the bowl and pour in the remaining egg mixture and beat for another minute.
Place the batter into the prepared tin and smooth the top using a spatula. Bake for 1 to 1.15 hours or until a skewer comes out clean when inserted in the middle. At around 45 minutes of the baking time, quickly open the oven and cover the tin with loose foil to help prevent the top of the cake from going too dark.
Let the cake cool in its tin on top of a cooling rack for 30 minutes before turning the cake out. Leave the cake upright on the rack until it has cooled down completely.
For the chocolate orange ganache - Roughly chop the chocolate into little pieces and place it in a medium heatproof bowl. Mix the cream and orange zest in a small saucepan over medium-low heat, cook just until small bubbles appear, but don't let it boil. Pour the cream mixture over the chocolate and leave it untouched for a minute. Stir the chocolate mixture well to combine. It should come together and look glossy. Leave the ganache uncovered at room temperature for an hour. If it's still too thin, place it in the fridge at 15-minute intervals, stirring each time until you get the desired or spreadable consistency. Don't freeze it, as it may cause the ganache to seize. Place the ganache over the cooled cake and use a spatula to spread it all over the top of the cake evenly.