Take the butter out of the fridge and set on the counter to bring it to room temp. Preheat the oven to 350° F and grease two 12-cupcake pans with either butter or cooking spray, or line with paper cupcake cups. In a medium bowl, sift together all of the dry cupcake ingredients (flour, sugar, cocoa powder, baking soda, baking powder, salt), and whisk to combine.
In a separate large bowl, whisk together the eggs, sour cream, canola oil, and vanilla extract until fully blended and creamy. Once combined, add in the dry ingredients and use a spatula or wooden spoon to mix. Fun step! Poor in that cup of piping hot coffee and stir until fully combined (it’ll take some working). Pour batter into the cupcake pans (each one should be about 2/3-3/4 full) and bake for 17-20 minutes or until a toothpick inserted into the middle of one of the cupcakes comes out clean. Remove from oven and let cool.
While the cupcakes are baking, make the fudge topping. In a large frying pan, melt the butter over medium heat. While melting, dissolve the instant coffee granules in the milk. Once butter is completely melted, toss in the cocoa powder and use a whisk to stir constantly (it will be very dry). Add the sugar and continue to push around the pan/stir for 20-30 seconds, then pour in the coffee+milk mixture, vanilla and salt and whisk rapidly, scraping sides of pan until everything is combined and smooth (this will take a minute or so). Continue to stir as it thickens, another 30 seconds, then, once it's fairly thick (not pourable), remove from heat and let cool (not chilled - it will be too hard to spread on the cupcakes!).
Once cupcakes are fairly cool, spoon about a teaspoon of the fudge topping on top of each one and spread so that each cupcake is capped in fudge sauce. Let cool completely while you make the buttercream.
Make the buttercream! In a large bowl, using an electric mixer, beat the room temp butter until creamy. Add in the powdered sugar 1/2 cup at a time and beat on low until combined. Then, in a small bowl or cup, dissolve the coffee granules in the vanilla extract. Pour vanilla + coffee mixture into the buttercream and continue to beat on medium high until fully smooth, and combined. Either use a spatula to spoon the frosting into a piping bag or simply frost each cupcake using a spoon or knife - however fancy you want to get! Frost cupcakes and then serve!