Preheat oven to 350 degrees F.  Butter two 6-inch cake pans and line bottom with parchment paper.  Set aside.

Sift together cake flour, cocoa powder, espresso powder, baking powder, baking soda and sea salt. Set aside.

 In the bowl of a stand mixer fitted with paddle attachment, beat butter on medium speed. Slowly add sugar. Continue to mix, scraping down bowl as needed to ensure throughout creaming. Add eggs one at time, mixing on low speed. Add egg white and vanilla extract.

Add 1/3 of dry flour mixture and mix on low speed. Add 1/2 of buttermilk and mix until incorporated. Follow with another 1/3 of flour, then remaining buttermilk. Finish with last 1/3 of flour mixture. Mix on low speed until there are no longer any dry streaks of flour.

Divide batter evenly between the two prepared cake pans. Bake for 40-45 minutes until toothpick inserted in center of cake comes out clean. Let cool in pan for 5 minutes. Unmold the cake and set on a wire rack to cool to room temperature.

Place chopped chocolate in a large bowl.  Bring heavy cream to a simmer.  Pour hot cream over chopped chocolate and cover bowl with plastic wrap.  Let sit for 3 minutes.  Remove plastic wrap and stir ganache until homogenous.  Cover with plastic wrap, allowing plastic wrap to touch chocolate to prevent condensation.  Every 30 minutes, stir ganache until it has thickened to frosting consistency (about 2 hours).

Melt sugar and water in a heavy bottomed sauce pot on high heat.  Lower heat and allow sugar syrup to color.  Remove from heat once syrup is aromatic and amber in color. 

Warm up heavy cream in the microwave for 45 seconds. Add about 1/3 of warm cream to sugar syrup and whisk. BE CAREFUL as the mixture will bubble and steam. Add the remaining warm cream and whisk until mixture has slightly cooled. Add butter and salt. Whisk until mixture is homogenized. 

Let cool to room temperature. Once cooled, sauce will thicken. To speed up the process, you can also place caramel in the fridge to cool faster.

Once cake is cooled, slice each cake into two equal slices. To build the cake, place one layer on a serving platter. Pour between 1 1/2 tablespoon to 2 tablespoons of caramel over the layer. Spread evenly. 

Place another cake layer on top and repeat until you have the fourth cake layer on top. 

Spread thickened chocolate ganache all over the cake. Pour remaining caramel sauce over the edges of the cake. Sprinkle Maldon sea salt over caramel, as desired.