Preheat oven to 350 F (175 C). Grease and line three 8-inch round cake pans.

In a large bowl, combine the dry ingredients: 2 cups flour, 2 cups granulated sugar, 3/4 cup unsweetened cocoa powder, 2 tsp baking powder, 1 1/2 tsp baking soda, and 1 tsp salt. Whisk until evenly mixed.

In a separate bowl, whisk the wet ingredients: 1 cup buttermilk, 1/2 cup vegetable oil, 2 large eggs, and 2 tsp vanilla extract.

Mix the wet into the dry until just combined, then add 1 cup boiling water carefully, stirring until the batter is smooth and thin.

Divide batter evenly into the three prepared pans. Bake for 30–35 minutes or until a toothpick comes out clean. Cool in pans briefly, then turn out and cool completely on wire racks.

In a heavy saucepan, melt 1 cup granulated sugar over medium heat, stirring gently until it reaches an amber color.

Carefully add 6 tbsp cubed unsalted butter and stir until melted. Slowly pour in 1/2 cup heavy cream, stirring until smooth. Stir in 1 tsp sea salt.

Let the caramel cool until slightly thickened but still pourable.

Beat together 1 cup unsalted butter and 1 cup creamy peanut butter until light and fluffy.

Gradually add 4 cups powdered sugar, 1/4 cup heavy cream, and 1 tsp vanilla extract, beating until smooth and spreadable. Add a little more cream if the frosting is too stiff.

Place one cake layer on a platter, spread a generous layer of peanut butter frosting, and drizzle a spoonful of the cooled salted caramel. Sprinkle a few crushed toffee bits.

Repeat with the second layer. Top with the third layer, cover the outside of the cake with the remaining peanut butter buttercream, smoothing with an offset spatula.

Drizzle additional salted caramel over the top and down the sides, then sprinkle the remaining crushed toffee.

Chill the cake for about 20 minutes before slicing to help the layers set.