A delightfully flavored mocha cake that is moist, soft and moderately sweet.
Preheat oven to 350 F and line a 9 x 13 baking pan with parchment paper. Dissolve 2 teaspoons of instant coffee powder in 1 teaspoon 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 sugar until pale and slightly thicker. Whisk in the water, oil and the reserved coffee liquid. Stir everything until incorporated.
Gradually add the flour mixture and whisk 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 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 coffee powder in 1 tbsp hot water and set aside to cool.
Make a double boiler set up: Bring a pot that is about 1/4 full with water to a simmer. Place a heatproof glass bowl that fits on top of the pot over it 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 8-12 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. Store buttercream in a tightly covered container overnight in a cool place or use immediately. You can refrigerate it if you are not using it for a couple of days, but you might have to re-whip it to get the spreading consistency.
Ice the cooled cake. Cake can be kept at room temperature for up to two days and should be refrigerated for longer storage.