Grease and flour* 3 9-inch baking rounds. Preheat the oven to 350°F.
In a mixing bowl, add flour, sugar, cocoa powder, baking powder and baking soda. Mix until combined.
On low speed add eggs, oil, milk, vanilla and espresso powder. Mix on medium speed for 1 to 2 minutes until the wet ingredients are fully incorporated.
Carefully add the boiling water and mix on low speed for 1 minute, scraping down the sides and bottom of the bowl.
Divide the batter evenly between the three pans. Bake for 20 to 22 minutes, rotating the pans halfway if you are using more than one shelf in your oven. Let cool completely before assembling.
In a large sauce pan, combine granulated sugar and corn syrup. Mix over medium-high heat until combined and mostly liquid. Cook for 10 to 12 minutes, without stirring, until mixture becomes a dark amber color.
Once a dark amber color is achieved, remove from heat. CAREFULLY pour in heavy cream and stir until smooth. Mixture will bubble and steam during this process.
Return to medium-high heat and cook until mixture reached 238°F on a candy thermometer, about 2 minutes.
Pour caramel into a heat proof bowl. Stir in kosher salt and let cool 15 minutes.
After the caramel is cooled, stir in the cold butter one tablespoon at a time. Add the next piece only after the previous piece is fully incorporated. Once all butter is incorporated, let cool another 5 to 10 minutes before decorating the cake.
Milk Chocolate Buttercream
Using a double boiler, melt the milk chocolate chips. Once melted allow to cool fully before incorporating into frosting, about 15 minutes.
In a small bowl, whisk together ⅓ cup + 1 tablespoon cocoa powder and ⅓ cup + 1 tablespoon warm water.
In a standing mixer, beat butter and a generous pinch of kosher salt on medium speed until pale and fluffy. Add confectioner's sugar and mix for an additional two minutes.
Slowly incorporate the melted milk chocolate and mix on medium speed for two minutes, scraping down the sides of the bowl. Add the cocoa powder mixture to the bowl and mix on medium speed a final two minutes until fully incorporated.
Transfer to a large piping bag fitted with an open tip. reserve ⅓ for an additional piping bag fitted with an open star tip.
Assembly for Salted Caramel Pretzel Cake
Pipe a small dollop of frosting on the cake stand or cardboard to prevent the bottom layer of cake from moving.
Pipe around the edge of the first layer. Add ½ cup of salted caramel inside the piped area and spread with a heated flat spatula or decorators tool to smooth out. It doesn't need to reach the edge completely, the weight of the top layers will smooth it out.
Sprinkle ½ cup of the broken pretzel pieces over the caramel layer. Repeat this process for the second layer. Top with the third layer of the cake.
Pipe the top of the cake and lightly around the outside. Smooth the frosting and scrape down the sides for a 'naked' cake look.
Working over a large plate or bowl, gently press broken pretzel pieces on the bottom ⅓ of the cake. Refrigerate for 10 minutes.
Using a small open tip, pipe the outer edge of the cake so the caramel cascades in a 'drip' style over the edge. Finish the top with various zigzag patterns. Sprinkle with remaining broken pretzel pieces. Refrigerate for 5 minutes to solidify the caramel before additional piping.
Using the reserved Milk Chocolate Buttercream fitted with the large open star tip, pipe small to medium sized stars along the out edge of the top of the cake. Finish with placing a full pretzel in-between every 2 to 3 stars.
Store at room temperature until service. Enjoy that day to avoid pretzels become stale or soggy from refrigerating overnight.
*Goop is an easy and fail proof alternative to greasing and flour baking pans.