For the Cake:
Preheat oven to 350F.
Line three 6" cake pans with parchment paper rounds. Grease the bottoms and sides lightly with butter, and then dust with flour. Tap the pans to eliminate any extra flour.
In a large bowl, combine the sugar, flour, cocoa powder, salt, baking powder, and baking soda. Using a whisk, stir to combine the dry ingredients. Add the lightly beaten eggs, canola oil and the milk and combine. Dissolve the espresso powder into the in the boiling water and then stir it into the mix. Your batter should now be quite liquid, like pancake batter.
Divide the batter equally into the three pans. Place into the oven and bake for 30 - 35 minutes. Remove from the oven and let cool for 10 minutes before removing from their pans. Let them finish cooling on a wire rack.
For the Caramel Sauce (this can be made up to three days ahead):
Place the corn syrup, water, and granulated sugar in a medium saucepan and stir to combine. Bring to a boil over medium heat. Once the mixture starts to boil, don’t stir it or the sugar could crystallize. You can gently swirl the pan every so often if the sugar begins to brown unevenly (be very careful, it’s HOT). Continue to boil until it thickens and turns a lovely brown caramel color (once it starts to color, it will darken quickly so don’t take your eyes off it). Turn off the heat and slowly stir in the cream, vanilla, salt, and lemon juice with a wire whisk. It will bubble up like crazy for a moment but just keep stirring. If the caramel clumps at all, place the pan back on the stove on low heat, and stir until it becomes smooth. Set to the side to cool completely.
For the Caramel Buttercream:
In a mixer, combine 2 sticks of softened butter, 4 cups powdered sugar, and 3/4 of the cooled caramel sauce (1/2 cup). Beat until smooth and fluffy.
Once the cakes have cooled completely, assemble them by placing a thick layer of frosting between each layer. Cover the top and sides with the rest of the frosting and smooth out with a spatula. Drizzle the left over caramel on top. Sprinkle with some flaked Maldon salt.