Make the cake: In a large mixing bowl, mix together the flour, sugar, baking soda and salt. In a separate bowl, dissolve the instant coffee in the hot water. Then add the cocoa powder and vanilla, whisk together, then pour into the dry mixture. Add the vegetable oil, chocolate chips and vinegar, and mix until everything is well combined.
Pour into a greased 9-inch round cake pan and bake at 375F for 30-35 minutes. Let cool in the pan for 10 minutes, then transfer to a wire rack and cool completely.
Make the buttercream: Place the butter in a large bowl and beat with an electric mixer until pale and fluffy. Add the confectioner’s sugar one cup at a time, then continue beating until fluffy. Add the cocoa powder and mix until combined. Place about ¾ of the buttercream in a piping bag fitted with a large, star-shaped piping tip.
Trim the edges off the cake to create clean edges. Using a plate or bowl as a guide is very handy! Then spread a thin layer of buttercream over the entire surface of the cake and transfer the cake to the fridge to chill, for about 10 minutes.
Transfer the cake to the serving platter of your choice and pipe swirls of buttercream over the entire surface. Top with birthday candles and enjoy!