Step 1. For the cake: Preheat the oven to 350˚. Coat three 8-inch round cake pans with cooking spray and dust with flour. Line the bottom of each pan with a round of parchment paper.
Whisk the granulated sugar, flour, baking powder, baking soda and salt in a large bowl. Set aside. Melt the butter in a small saucepan over medium-low heat. Meanwhile, combine the instant coffee and boiling water in a medium bowl and stir to dissolve.
Once the butter has melted, add the coffee mixture to the saucepan. Let it come to a boil for about 10 seconds, then remove from the heat and set aside.

Step 2. In a separate medium bowl, whisk the buttermilk, eggs and vanilla until well combined. Pour the butter-coffee mixture into the flour mixture and stir gently.
Add the buttermilk-egg mixture and stir gently until well combined. Divide the batter among the prepared pans.
Bake until a toothpick inserted into the centers comes out almost clean, 23 to 25 minutes. Allow the cakes to cool for 15 minutes in the pans on a rack, then remove the cakes from the pans, remove the parchment and let cool on the racks.

Step 3. For the frosting: Beat the butter in a large bowl with a mixer on high speed until light and fluffy, about 1 minute.
Add the powdered sugar and salt and beat on low speed to incorporate. Stir the instant coffee into the heavy cream in a small bowl, then pour it into the mixer.
Beat on low speed until combined, then beat on high speed until light and fluffy, about 2 minutes, adding a little more cream if needed