Chocolate cake rolls are deep and rich two-layer rolls that keep well in the freezer for a while, yet are fresh every time you serve them. So good you'll want to make two batches at once.
Chocolate layer: in a large bowl, combine walnuts, chocolate and confectioner's sugar. Mix well with your hands until integrated. Add vanilla extract (or rum) and mix again. Finally, add milk and mix vigorously until you get a semi-hard thick mixture.
Divide into 4 equal parts. Work each into a smaller rectangle, the thickness of 2-3 fingers. Set aside.
Yellow layer: fill a large pot with water ⅔ in, bring to a boil on high, then reduce temperature to medium. Place a smaller pot in the large one making sure it fits inside, while the upper portion stays on the outside. (Essentially, the smaller pot is half-way in.)
Put yolks in the smaller pot, add sugar, and mix with a hand-mixer until egg yolks become light yellow. Hold the pot with the other hand the best you can, making sure: 1. you're mixing everything, and 2. you're being careful not to flip the pots over and burn yourself. (This part is crucial as you're processing the egg yolks.)
Add butter to the pot, and continue mixing. Once the mixture is completely smooth (3-4 minutes), remove off heat.
Combining: place a piece of baking paper on a working area and sprinkle with confectioner's sugar. Take one chocolate layer and spread it out into a rectangle with a rolling pin until it's the thickness of a finger (approximately). Take ¼ of the yellow layer and spread over the top evenly. Roll the cake in starting with the longer end. Cover in baking paper and leave in the fridge or freezer depending on when you plan to consume it. Repeat steps for the remaining 3 rolls.
Before serving cut each roll into 1-inch pieces.
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Notes
You will get 4 chocolate cake rolls with these ingredients. You can easily halve it. One roll can serve 4-5 people.