Preheat oven to 350°F / 175°C. Line the bottom and the sides of two 8" (20cm) baking pans with parchment paper. Set aside.
Cake: In a medium bowl combine flour, cocoa, sugar, baking powder, salt, and espresso powder and stir to combine. Set aside. In a large mixing bowl, using an electric mixer fitted with a whisk or paddle attachment, beat oil, eggs, buttermilk, and vanilla just until combined. Stir in dry ingredients to combine. Slowly mix in hot coffee until combined. The batter will be very thin. Divide the batter into the two prepared baking pans equally. Bake for 20-25 minutes or until a toothpick in the center comes out clean. Let cool in the pan for about 20 minutes. Then remove from pan and transfer to a cooling rack. Let cool to room temperature.
Frosting: In a large mixing bowl, using an electric mixer fitted with a whisk attachment, beat mascarpone on medium-high speed until creamy, about 1 minute. Sift in powdered sugar and mix until combined and smooth, about 1 minute. Add chilled coffee and mix until incorporated and smooth, about 1 minute. Add heavy cream and mix until creamy, about 1 minute.
Assemble the cake: Cut a thin layer off the tops of your cakes to create a flat surface. Then cut each cake in half horizontally. Place one cake layer on a cake stand. Spread ¼ of the frosting over the cake. Repeat two more times. Place last cake layer on top and lightly frost the outside and the sides of the cake with the remaining frosting. Chill at least 4 hours in the fridge or overnight.
Ganache: Place chopped chocolate in a large heatproof bowl. Place heavy cream and corn syrup in another heatproof bowl and microwave until very hot, almost boiling for about 1-2 minutes. Pour over chocolate and let stand 1-2 minutes. Whisk until smooth, and the chocolate is completely melted. Let stand for about 10-15 minutes. Test the consistency of the glaze down the side of the bowl. It should drip. The longer it stands, the thicker it gets. If it is too firm, reheat it for a few seconds in the microwave. Once it has the desired consistency, it's ready to spoon on top of the cake.
Remove cake from the fridge and spoon the glaze little by little around the edges and let it drip down the sided. Do this with a small spoon and do one drip at a time. Then spoon or pour the rest of the glaze on top of the cake (maybe you don't need all of it) and level with an offset spatula. Place cake in the fridge for 15 minutes so that the ganache sets. Then serve. Store in an airtight container in the fridge for up to 3 days.