Preheat your oven to 180°C (160°C fan) and line an 8x10-inch tin with baking paper.
In a large bowl, cream the butter and sugar together. Then stir in the orange extract and vanilla extract.
Stir the eggs, flour, cocoa powder and milk in, a little bit at a time, until you have a smooth, thick batter. Then stir in the orange zest.
Spread the batter out in your lined tin and bake for 25-30 minutes, until a toothpick comes out clean.
Let the cake cool in the tin for 10 minutes then gently flip it out onto a wire rack to cool completely.
Topping
Melt 100g of Terry's Chocolate Orange and set it aside until it's completely cool to the touch.
In a medium bowl, mix the butter, icing sugar and cocoa powder together until smooth, adding the milk to loosen it as needed. Then stir in the cooled melted chocolate.
Spread the icing over the cake then top with Terry's Chocolate Orange segments and sprinkles.