Preheat the oven to 350F/177C and line three, 8-inch (20-cm) cake pans with parchment paper and spray the sides with a baking spray.
Prepare the cake layers first: in a large mixing bowl, combine the eggs, dulce de leche, vanilla and sugar. Beat on medium speed for 2 to 3 minutes until the mixture is smooth. Next, add in the buttermilk and oil. In a small ramekin, combine the instant coffee and hot water; stir to dissolve, then add to the batter. Beat the batter for another minute, until it’s smooth.
In a separate bowl, combine the dry ingredients: flour, baking soda, baking powder and salt. Sift the dry ingredients into the cake batter and beat for about 1 minute, just until combined.
Divide the cake batter evenly between the three pans and level the batter with an off-set spatula. Bake the cake in the preheated oven for about 24 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake layers from the oven, out of the pans and onto a wire rack to cool completely.
Making the Filling:
Next, prepare the filling. Combine the sweetened condensed milk and dulce de leche in a small bowl, then whisk until creamy. Fold in the diced pecans with a small spatula. Set the filling aside until ready to use.
Making the Caramel Frosting:
For the frosting: This recipe makes quite a bit of frosting. If you typically don’t like a lot of frosting, prepare just half of the recipe. I recommend making this frosting while the cake layers are baking and cooling to allow 1 hour of chill time for the frosting. In a large mixing bowl, combine the softened mascarpone with dulce de leche. Mix on medium speed for a couple minutes until the mixture is smooth. Next, add in the sweetened condensed milk and vanilla. In a small ramekin, combine the instant coffee and hot water; stir to dissolve, then add to the mixture. Mix the frosting for 1 minute, until it’s smooth and uniform.
Pour the chilled heavy cream into a separate mixer bowl and whisk for about 4 minutes, until stiff peaks form. Start the mixer on low speed, then gradually work up to high speed once the cream thickens. Pour the prepared mascarpone and dulce de leche mixture into the whipped cream. Mix on high speed for 15 to 20 seconds, just until the two mixtures are combined. DO NOT OVERMIX! Overmixing can cause the frosting to separate. If there is any whipped cream that did not mix with the mascarpone, use a spatula to fold it in. Place the frosting into the refrigerator to set for one hour and keep it chilled when not using.
Assembling the Golden Key Cake:
To assemble the cake, first make sure the cake layers are completely cooled. Use a long, serrated knife to level off the top of each layer. Split the prepared sweetened condensed milk and pecan filling between two cake layers and very gently spread the filling to the edges. Watch my video recipe to see how it’s done.
Spread a generous amount of the prepared frosting between each cake layer and frost the outside of the cake with a thin layer of crumb coat frosting. Refrigerate the cake for 15 to 30 minutes, then apply the final coat of frosting. Garnish the top and sides of the cake with the diced pecans by gently pressing them into the sides and sprinkling on top. If desired, transfer any remaining cream into a pastry bag and pipe dollops of cream on top; I used Ateco tip #847.
Refrigerate the cake for at least 12 to 24 hours before enjoying. Keep the cake refrigerated when not serving.