In a small pot, boil about 1" of water. Meanwhile, in a large heat-safe bowl that fits perfectly on top of the pot without touching the water, whisk together the egg yolks and sugar.
When the water comes to a boil, turn off the heat and place the bowl on top of the pot. Continue whisking occasionally until the sugar has dissolved and the mixture becomes thicker and pale in colour, about 4-5 minutes. Remove the bowl from the pot.
Immediately add mascarpone cheese into the warm egg mixture and whisk until completely combined and smooth.
In a large mixing bowl, add whipping cream and vanilla extract. Use an electric hand mixer to beat until it becomes soft peaks.
Add the whipped cream to the mascarpone mixture and fold to combine until no streaks remain.
In a shallow bowl, stir together espresso powder, hot water, and coffee liqueur.
Very quickly dip a biscoff cookie into the espresso mixture, just enough to cover both sides. Place the soaked biscoff cookie in the bottom of your serving container. Repeat until you've arranged a double layer (single layer if you prefer) of biscoff cookies in the container (6 cookies per layer for my small container).
Add a generous layer of mascarpone cream on top of the biscoff layer and use your spatula to push the cream into the edges. Repeat with another layer of soaked biscoff cookies and again with another layer of mascarpone cream.
Use a bench scraper to scrape off excess cream from the top of the container. Chill tiramisu in the fridge for at least 4 hours or preferably overnight.
Before serving, use a fine mesh sieve to dust the top of the tiramisu with cocoa powder and place a biscoff cookie on top.