1. Preheat the oven to 350F. Butter and flour 3 8-inch cake pans and set aside.

2. Beat the butter and sugar together for 10 minutes, until light and airy. Meanwhile, dissolve the espresso in the boiling water and set aside. Sift the flour, baking powder, cocoa powder and salt and set aside.

3. Once the butter is fluffy add the eggs and mix well. Then add the rum, milk and espresso and beat until incorporated.

4. Turn the mixer to low and slowly add in the flour mixture until just combined. The batter will be very fluffy. Evenly distribute it amongst the three baking pans and bake for about 35 to 40 minutes, or until a cake tester inserted comes out clean. Allow the cakes to cool for about 10 minutes in their pans, then remove them and place them on wire racks to cool completely.

5. To make the buttercream: Combine the egg whites, sugar and cream of tartar in a heatproof bowl and warm them over simmering water for about 6 minutes, or until warm to the touch. Meanwhile, melt the chocolate together in the microwave and stir together. Set aside.

6. Place the cooled egg whites in a mixing bowl and whip until they hold stiff peaks. Then beat it the butter in small pieces, followed by the salt, melted chocolate and hazelnut extract. The frosting should be fluffy and light!

7. Frost the cake between each layer and all along the outer edges and top. Then, scatter the hazelnuts all along the top of the cake and press them along the sides as well. Serve!