1) Preheat the oven to 180°C/gas mark 4. Butter and flour a 30cm x 45cm x 1 cm sheet pan.
2) In a medium bowl, sift together the flour, cocoa, baking soda, and salt.
3) In the bowl of an electric mixer fitted with a paddle attachment, cream the butter and sugars on high speed until light, approximately 5 minutes. Add the eggs and vanilla and mix well. Combine the buttermilk, sour cream, and coffee. On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture. Mix the batter only until blended.
4) Pour the batter into the prepared sheet pan, smooth the top with a spatula, and bake in the center of the oven for 25 to 30 minutes, or until a toothpick comes out clean. Cool to room temperature before frosting.
For the orange buttercream:
1) Combine the sugar with 160ml water in a medium heavy-bottomed saucepan and, without stirring, bring to a boil. Cover the saucepan and allow the mixture to boil until the sugar dissolves. Uncover and continue boiling until the mixture reaches 115°C on a thermometer. Pour the syrup into a heat-proof measuring cup.
2) Place the egg whites and cream of tartar in the bowl of an electric mixer fitted with the whisk attachment and beat on high speed until the eggs form stiff peaks. With the mixer on high speed, slowly pour the syrup into the egg whites. Continue beating on high speed until the mixture is absolutely at room temperature, about 10 to 15 minutes.
3) With the mixer on medium speed, add the butter, 1 tablespoon at a time, and then add the vanilla and liqueur. (If the mixture becomes runny, the meringue was too warm and the butter melted. Chill slightly and beat again.) Add the food colouring and combine.