Preheat oven to 350° Fahrenheit.
Spray two jelly roll pans (10 x 15 inch) with a flour baking spray or line with parchment paper.
Melt the 4 sticks of butter in a medium saucepan.
Add the 6 Tbsp of instant coffee and then add the 2 cups boiling water.
Let this mixture bubble for about 5 seconds, then turn off the heat.
In a separate large bowl, whisk together the flour, sugar, and salt. Then pour the hot mixture over the dry ingredients and stir to combine.
In another medium bowl, mix together the buttermilk, eggs, baking soda, and vanilla extract. Pour this mixture into the ingredients in the large bowl and stir together. Mix until fully incorporated.
Pour this batter into the two jelly roll pans, dividing the batter equally between them both. Bake for about 20 minutes.
Take the pans out of the oven and let the cakes cool in the pans for about 10 minutes. Then turn the pans over on to a clean, prepared surface, and remove from the jelly roll pans. Gently peel off the parchment paper (if used) and allow the cakes to cool completely.
While the cake is cooling, begin preparing the filling.