A delightfully flavored mocha cake that is moist, soft and moderately sweet.
Preheat oven to 350 F. Line a 9 x 13 baking pan with parchment paper that covers the bottom and extends up the sides of the pan. Dissolve 2 teaspoons of instant coffee powder and 2 teaspoons hot chocolate powder in 2-3 teaspoons hot water. Set aside to cool.
In a large bowl, combine flour, salt, and baking soda. In a separate mixing bowl, whisk together egg yolks and 1 cup of sugar until pale and slightly thicker. Whisk in the water, oil and the reserved coffee/chocolate liquid. Stir everything until incorporated.
Gradually add the flour mixture and beat with an electric mixer just until the mixture is smooth. In a bowl of a stand mixer (or use a handheld mixer), Whisk the egg whites and the cream of tartar until soft peaks form. Gradually add 1/2 cup of sugar until firm peaks form. Gently fold this meringue to the yolk batter in three additions using a rubber spatula. Pour batter into the prepared baking pan and bake for 30-35 minutes. A toothpick inserted at the center of the cake should come out clean. Let cake cool slightly in the pan. Then transfer to a cooling rack to cool completely.
Dissolve 2 tbsp instant coffee powder and 1 tbsp hot chocolate powder in 2 tbsp hot water. Set aside to cool.
Make a double boiler set up: Bring a pot that is about 1/3 full with water to a simmer. Place a heatproof glass bowl on top of the pot (it should fit snugly) and add the egg whites and sugar to the bowl. Whisk the whites and sugar together continuously until is thick, opaque and the temperature reaches 160 F. Whisk it for 3 more minutes and turn off heat. This should take between 10-15 minutes.
Transfer the mixture to the bowl of stand mixer and using the whisk attachment, whisk the mixture using high speed until it forms firm peaks and the meringue is cold to the touch. Add the softened butter bit by bit. The mixture will be curdly at first and then it will turn to a smooth, fluffy buttercream. Add the coffee flavor and whisk for about a minute, until the color is uniform.
Coat the top and sides of cake with the buttercream. The meringue icing is enough to coover the enitrre cake with sopme piping along the edges and the bottom borders. I ised Wilton 1M to pipe the sides and edges. Cake can be kept at room temperature for up to two days and should be refrigerated for longer storage.
If you are preparing the swiss meringue buttercream ahead, store it in the fridge for up to three days until you are ready to use it. You need to let it come to room temperature and re-whip it with an electric mixer to get it into a spreading consistency.
The finished cake can be at room temperature, covered for up to 2 days. Refrigerate it for longer storage.