Set up a bain-marie by filling a medium saucepan with water and bringing it to a gentle boil, then reduce to a simmer. In a heatproof bowl, combine the butter, sugar, egg, honey, and heavy cream. Place the bowl over the simmering water, ensuring it doesn’t touch the water directly. Whisk constantly until the mixture turns creamy and light in color, about 5 minutes. Remove from the heat and allow it to cool slightly.

In the bowl of a stand mixer, sift together the flour, baking soda, and cocoa powder. Gradually fold in the warm butter mixture using a spatula until the dough comes together. Divide the dough into seven equal portions, approximately 130 grams each. Wrap each portion in plastic wrap to prevent it from drying out and let it rest for 5 minutes.

Preheat your oven to 180°C (356°F) and position the rack in the center. Roll each dough portion on a parchment-lined surface to a thin, 2-3 mm thickness, slightly larger than a 6-inch round pan. Use the pan as a guide, pressing gently into the dough, and trim the edges with a knife or pizza cutter to form a perfect circle.

Transfer the parchment paper with the dough circle onto a baking sheet and bake for 4-5 minutes until golden but still soft to the touch. Avoid overbaking. Repeat with the remaining dough portions, saving the scraps to bake as an additional disk for decoration. Allow the baked disks to cool on a wire rack.

Place the cans of sweetened condensed milk, labels removed, in a deep saucepan filled with water. Ensure the cans are fully submerged. Bring to a boil, then reduce to a simmer, and cover. Cook for 2 hours, ensuring the water level stays above the cans. Once done, turn off the heat and allow the cans to cool in the water until room temperature—never open hot cans, as the contents can cause severe burns.

Once cooled, open the cans and set the caramelized condensed milk aside. In a stand mixer, beat the softened butter on medium speed for about 10 minutes until it turns pale and fluffy. Add the dulce de leche and ground walnuts, mixing until fully incorporated. Divide the buttercream into seven portions, reserving them for the assembly.

In a small bowl, combine the sugar and dark rum. Add the boiling water and stir until the sugar dissolves completely. Set aside to cool.

Using a food processor, blend the last baked disk of honey cake along with the walnuts into fine crumbs. These will serve as the cake's decorative coating.

Place a piece of parchment paper on your serving platter and set the first honey cake disk on top. Use a syringe or measuring spoon to evenly distribute 30 ml of the sweet rum syrup over the surface of the disk. Spread one portion of the dulce de leche buttercream evenly over the moistened disk using an offset spatula.

Repeat this process with the remaining cake disks, stacking them as you go. Once the final disk is in place, cover the entire cake with the remaining buttercream. Press the prepared decorative walnut and cake crumb mixture onto the sides and top of the cake, using a spatula to gently press the coating into the buttercream.

Place the assembled cake into a covered cake container and refrigerate for at least 24 hours to allow the flavors to meld. To serve, slice the cake with a sharp dessert knife into 2-inch portions, revealing the stunning layers.