Special equipment: Special equipment: a platter or large plate, (individual plates, or mini glasses.) Candy thermometer. Make the espresso coffee, and mix with 1 tsp sugar while hot. The end result should be to your liking; not too strong, or too weak, is best.
Set up a double boiler (if you don't have one) by placing a glass bowl over a bit of hot water (the bowl should not touch the water) over low heat. Beat the egg yolks and remaining sugar off the heat for a few minutes, then transfer the bowl over to the barely simmering water in the pot until the temperature reaches 150 degrees F, or 65 C. Continue beating for 2 minutes, then allow to cool, stirring occasionally so it doesn't create a skin.
Add the mascarpone and speculoos butter to the egg and sugar mixture, and mix well.
Pour some cold coffee into a bowl. Be sure it's not hot or the biscuits will absorb too much liquid.
Start dipping the biscuits into the coffee, then place each one on the platter (or in the dish) on individual plate, or break so they fit into mini glasses. Next, spread or pipe some mascarpone/egg mixture over the soaked biscuits. If making one large dessert, place half of the creamy mixture over the layer of biscuits.
Repeat with the other half of the biscuits for the second layer.
Finally, spread with the rest of the filling, cover with cling film and refrigerate for at least 4 hours, but preferably overnight.
When ready to serve, remove from the refrigerator, sprinkle with speculoos crumbs and dust generously with cocoa (from a sieve).
To serve, slice into even pieces, wiping the knife in between each cut to produce clean edges, and serve using a metal spatula (for best results).