Chocolate Mango Cake

0 Preheat oven to 350 degrees F / 180 C. Line an 8 inch round baking dish with parchment paper and set aside.
1 Whisk together the sour cream, oil, eggs and vanilla extract and set aside.
2 In a separate mixing bowl whisk the brown sugar, salt, baking powder and soda, cocoa powder and flour until well combined.
3 Pour the wet ingredients into the dry ingredients and mix until combined.
4 In a separate bowl mix the boiling water and instant coffee.
5 Stir the coffee mixture into the cake batter and mix until incorporated.
6 Pour the cake batter into the prepared tin and bake for 45-50 minutes or until an inserted skewer comes out clean. If the cake is starting to brown before it is cooked cover it with foil to prevent it from burning.
7 Allow to cool in the cake baking dish for 10 minutes before removing and cooling completely on a cooling rack.
8 While the cake is cooling prepare the buttercream filling: Whip the pasteurized egg whites with the powdered sugar for 3 minutes. Add half the butter, whip until smooth. Add remaining butter and mango extract, whip for 4-5 minutes. Adjust the flavor to suit by adding an additional tsp of mango extract if desired.
9 Add a drop of orange food coloring and mix thoroughly if using.
10 Slice the cake in half with a serrated knife. 
11 Spread ½ of the buttercream, in an even layer, over the cake base. Add the top layer of cake, finish off with remaining buttercream and sliced mango if using.
12 Chill for 1 hour before serving.