Preheat the oven to 350 F. Butter and flour 3 6-inch round cake pans and line the bottoms with parchment paper. 

Melt the butter in a small saucepan or in the microwave, checking and stirring every 30 seconds until completely melted. Set aside to cool.

In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, and salt. 

In a separate bowl, combine the wet ingredients and whisk them until smoothly incorporated. Add the wet ingredients to the dry, and stir until just combined.

Divide the batter evenly among the three tins, using an offset spatula to smooth and level the batter. Bake for 22-25 minutes, or until a toothpick inserted comes out with a few crumbs.

Remove the cakes from the oven and run an offset spatula around the edge. Allow the cakes to cool for at least ten minutes before trying to remove them from the pans. Then let them finish cooling completely before wrapping and refrigerating or freezing them, depending on how far in advance you are preparing your cake. 

In a small heavy-bottomed saucepan combine sugar, water, corn syrup, and salt. Set over med-high heat and bring to a boil. Though tempting, do not stir your caramel, this causes crystallization and will ruin your batch if you deem it necessary swirl the pan from time to time, but even this can be risky. Continue to cook the caramel until it reaches a nice deep amber, this should take about 10-15 minutes. Watch it closely as perfection in caramel sits very near totally burnt. 

Remove the caramel from the heat and stir in the heavy cream. Do this slowly as the caramel will react violently to the introduction of the cold cream, hissing and bubbling furiously. Just continue to stir briskly. The caramel may seize up slightly, but just keep stirring, it will melt and smooth out. Add the bourbon, butter, and vanilla and stir until everything is fully incorporated and melted. 

Transfer the hot caramel to a jar or bottle and allow to completely cool covered in the refrigerator. Once thoroughly chilled the caramel will become quite thick. 

Combine the egg whites and sugar in the bowl of a standing mixer. Set over the same pot of simmering water you used for the chocolate. Using a wire whisk beat the egg whites over the water until they reach 120℉, or until a small amount of the mixture rubbed between your fingers feels smooth and not gritty.

Once this texture is achieved transfer your bowl to the standing mixer and beat on high until the meringue is completely cool, this takes several minutes, be patient.

When the meringue has cooled to room temperature begin adding the softened butter, little by little. Only adding more after the previous one has been incorporated. Add the caramel and stir to combine, adding more if necessary. 

Split the thoroughly cooled cakes. Place the bottom layer on top of a cardboard cake round, brush with simple syrup, drizzle with bourbon caramel, top with buttercream, sprinkle with toffee bits, then add the next layer of cake. Repeat with all the layers, crumb coat with remaining buttercream refrigerate until chilled. Ice with more buttercream. The decoration is, of course, Groom's choice.