Combine the hot water, coffee and sugar in a small bowl. Stir well and set aside to cool.
Now in a mixing bowl, use a spoon or spatula to whip the mascarpone until smooth and light. Place the bowl in the fridge.
In another mixing bowl or in the bowl of a stand mixer, beat the chilled cream with the sugar and vanilla on high speed until it thickens and forms soft, rounded peaks. Add to the mascarpone and fold gently to combine until smooth. Try not to deflate the mixture. If you feel the mascarpone isn't blending in properly, you can use a hand mixer or stand mixer on low speed to gently beat the mixture for a few seconds. Place this mixture in the fridge.
Now get out an 8x4 dish that's at least 2 inches high. Dip each biscuit one at a time in the coffee syrup from step 1. Dip for hardly a couple of seconds, because the biscuits can go soggy quickly.
Place each dipped biscuit in a single layer in the dish, breaking them if needed to fit the corners. Spoon half the cream mascarpone mixture on top. Repeat with a second layer of coffee soaked biscuits and top with the remaining cream mascarpone mixture. I used about 15 biscuits in all (see notes).
Dust the top with cocoa powder, then place the dish in the fridge for at least 4 hours. Slice into pieces and serve chilled. Any leftover coffee syrup can be spooned over the edges of individual servings since sometimes the biscuits may absorb the liquid unevenly. Tastes best on day one though you can store it for 2 days in the fridge. Happy tiramisu to you!