Make the coffee fudge by first combining 6 tbsp of strong coffee, 4 tbsp of unsweetened cocoa powder, 1 pinch of kosher salt and 2 oz of dark chocolate (roughly chopped) in a medium size bowl. Whisk together vigorously then set aside.
In a saucepan over medium-high heat, combine 1/2 cup of heavy cream, 4 tbsp of granulated sugar and 4 tbsp of light corn syrup. Stir frequently to ensure the sugar has melted and that the cream does not burn. Once the mixture comes to a boil (bubbles break across the surface), pour immediately over the coffee / chocolate mixture and let sit for a full minute. Once the minute has passed, whisk together the combined mixture until smooth (3-5 minutes).
Let chocolate coffee mixture cool (which will look more like chocolate milk than fudge) to room temperature before placing in the fridge for 45 minutes to 1 hour. Once properly cooled, the combined mixture will thicken to a fudge consistency.
While the fudge cools, make the condensed milk cream. In a stand mixer bowl that has been chilled in the freezer for 10-15 minutes, combine 2 1/2 cups of chilled heavy cream and 2 tbsp of vanilla extract. Using the whisk attachment, whisk on the lowest speed for 1-2 minutes before increasing the speed up to medium. Whisk until the mixture has reached soft peaks.
Once the mixture has reached soft peak consistency, add in 1/3 cup of powdered sugar and the pinch of kosher salt. Mix on medium speed until well incorporated.
Take the bowl off the stand mixer and using a silicone spatula, slowly incorporate the condensed milk into the cream by drizzling 1/4 cup of condensed milk slowly before folding it gently into the mixture. This will essentially swirl condensed milk throughout the cream. Set aside in the refrigerator until ready to use.
To assemble the cake once the fudge and condensed milk cream are ready, line a 8.5" by 4.5" loaf pan with plastic wrap both horizontally and vertically to allow easy removal of the cake once completed (usually 3-4 layers to be safe). Allow enough overhang over the sides so that the cake can be easily lifted out and wrapped with excess plastic wrap.
On the bottom of the loaf pan, layer one single layer of graham crackers, making sure the loaf pan is completely filled (cutting pieces of graham crackers to fill any spots).
Next, spread a layer of coffee fudge over the graham crackers. Then, spread a layer of condensed milk cream (reserve at least 1 cup of whipped cream for serving, keep in fridge until needed). Drizzle a few tbsp. of the remaining condensed milk and then top with chocolate shavings. Continue to alternate layers starting with the graham crackers, coffee fudge, condensed milk cream, condensed milk drizzle and chocolate shavings until you’ve reached the top of the loaf pan.
Wrap tightly in plastic wrap and refrigerate overnight or at least 12 hours. Once you’re ready to serve, unwrap the icebox cake and flip over onto a serving platter. Top the cake with the remaining whipped cream and top with chocolate shavings.