Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line with parchment paper.
Step 2: In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.
Step 3: In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract.
Step 4: Pour the wet ingredients into the dry ingredients and mix on low speed with an electric mixer until just combined. Slowly add the hot coffee and mix until the batter is smooth. The batter will be thin.
Step 5: Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.
Step 6: While cakes cool, prepare the Fudge Frosting: In a medium saucepan, melt the butter over medium heat. Add the cocoa powder and milk, whisking until smooth. Bring the mixture to a gentle boil, stirring constantly, then remove from heat.
Step 7: In a large bowl, combine the sifted powdered sugar with the hot cocoa mixture. Add vanilla extract and a pinch of salt. Beat with an electric mixer on medium-high speed until light and fluffy, about 5-7 minutes. If the frosting is too thick, add a tablespoon of milk; if too thin, add a tablespoon of powdered sugar.
Step 8: Once the cake layers are completely cool, level them with a serrated knife if necessary. Place one cake layer on your serving plate or cake stand. Spread about 1.5 cups of frosting evenly over the top.
Step 9: Carefully place the second cake layer on top. Frost the top and sides of the entire cake with the remaining fudge frosting. Decorate as desired.
Step 10: Let the frosting set slightly before slicing and serving. Enjoy your decadent chocolate fudge layer cake!