In a small bowl mix the vanilla and instant coffee and set aside.
Combine the sugar, egg whites, and salt in a stand mixer bowl. Place the stand mixer bowl over a saucepan filled with about 1 inch of simmering hot water, making sure the bottom of the bowl isn't touching the water. Whisk the mixture constantly, until the sugar dissolves and the mixture is starting to thicken up and become foamy. The temperature should be at around 160F.
Transfer the bowl to a stand mixer and start whipping the egg mixture right way. We want the meringue to reach stiff peaks and cool down at the same time. You will know the meringue is cool enough when the outside of the bowl feels cool to the touch.
Slowly add in the butter about 1/2-1 tablespoon at a time while mixing at medium speed. Make sure to let the butter fully incorporate before adding in the next tablespoon.
Now time to add in the cooled-down melted chocolate, vanilla coffee mixture, and salt then mix again with a paddle attachment on medium speed until combined. Make sure to scrape down the sides as you go.
(Optional if you want your buttercream to be darker in color) Sift in the cocoa powder and mix with the paddle attachment until combined.
Whip one last time on medium-high speed for 30-40 seconds and there you have it, the most perfect and luscious small-batch chocolate frosting.
This small batch chocolate Swiss buttercream makes 450g of buttercream.