Place sugar and water into a medium pot, stir to combine, but to not stir from this point forward. Cook over high heat, washing down the sides of the pot with a pastry brush dipped in water as needed to prevent crystals.
Cook until desired color of caramel is reached (amber) and immediately remove from heat. Very slowly pour in (room temperature) heavy cream while whisking quickly. The mixture will bubble up (a lot) and boil. Add (room temperature) butter, return to heat, and bring back to a boil. Simmer for 2 minutes whisking constantly.
Remove from heat and let cool to room temperature (about 2 hours). Transfer to a container and place in fridge to thicken.*
Preheat oven to 350F and grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, salt, and spices. Set aside.
Using a stand mixer fitted with the paddle attachment, beat butter until smooth. Add sugars and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add molasses and vanilla and mix until incorporated.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans. Smooth the tops with a spatula.
Bake for approx. 30-35 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Caramel Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a pot with 1-2" of simmering water and whisk constantly until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (about 3mins).
Place bowl on your stand mixer and whip on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.
Add cooled caramel and whip until smooth.***
Preheat oven to 350F and line a baking sheet with parchment.
Place cookie crumbs and melted butter into a medium bowl, stir until it just starts to clump. Spread out on prepared baking sheet and bake for 8-10mins.
Cool completely before using on cake. Break up crumble if needed.
Place one layer of cake on a cake stand or serving plate. Top with 2/3 cup of frosting. Dollop 1-2 Tbsp caramel on top and swirl into the buttercream using a toothpick. Sprinkle with 1/4 cup gingerbread cookie crunch. Repeat with next layer.
Place the final layer on top and do a think crumb coat on the cake. Chill for 30mins.
Using a small spoon, place dollops of caramel around the top edges of the cooled cake, allowing some to drip down.****
Fill in the top of the cake with more caramel and spread evenly with an offset spatula. Let the caramel set for a few minutes. *****
Do a rope border using a 1M tip and fill the center with more gingerbread streusel.
* The caramel can be made the day before and either left on the counter overnight or in the fridge if longer. Bring to room temp again before using.** Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen.*** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.**** If caramel has thickened too much, microwave in 5-10 second intervals until it's pourable (but not warm). You want the caramel to be rather thick for the drips. It will continue to drip down.***** Don't use too much caramel to fill in the top. I literally just pulled inward the top of the dollops to fill it in.You will have some caramel left over. The buttercream recipe makes enough to fully frost the cake.