When the mixture comes to a boil, reduce the heat to medium and watch carefully (no walking away!). The syrup will begin to turn golden at the edges; swirl the pan gently without lifting it off the heat to even out the color.
The syrup will continue to darken to light amber; as soon as you see any darker streaks, remove the pan from the heat. Don't let the sugar syrup become too dark — it'll continue to cook a bit even after it's off the heat. (The darker the syrup, the richer its flavor; but bitter/smoky notes start to creep in if you let it darken beyond medium amber.)
As soon as you remove the caramel from the heat, whisk in the butter. Gradually add the heavy cream, stirring until it’s incorporated. Add the vanilla, stirring until the sauce is smooth.
Let the sauce cool slightly, then transfer to a heatproof container and let stand for about 30 minutes at room temperature. Transfer to the refrigerator, covered, for 2 to 3 hours, until completely cool; it will firm up but will not solidify.
To make the mascarpone filling: In the bowl of a stand mixer fitted with the whisk attachment or working in a large bowl with an electric hand mixer, combine all the filling ingredients. Whisk at medium-high speed until medium-soft peaks form, 3 to 5 minutes.
To soak the ladyfingers and assemble: Have on hand a 9" square glass serving dish or metal pan, ready to construct the tiramisu. (There’s no need to line or grease the pan unless you want to remove the full tiramisu at once for serving, in which case, line the pan with a parchment sling.)
In a shallow bowl, whisk the warm milk with 2 tablespoons (25g) of the sugar and 1/2 teaspoon espresso powder until the sugar dissolves.
Working with one ladyfinger at a time, quickly dip both sides into the soak, about 5 seconds per side; the ladyfingers should feel soft and saturated but not mushy. Place the soaked cookies in the bottom of the pan, creating a single layer as you work. (Store-bought lady fingers should fit neatly into a 9" square pan. If using homemade ladyfingers, you may need to trim them slightly so that they form an even layer with no gaps.)
Once the ladyfingers cover the bottom of the pan, drizzle a scant 1/2 cup (110g) of caramel sauce evenly over top, then use a small offset spatula or the back of a spoon to spread it into a thin layer. Dollop about half of the mascarpone filling (1 3/4 cups or 265g) over top, then spread evenly. Once again, drizzle a scant 1/2 cup (110g) of caramel sauce over the filling and spread for even coverage — it’s OK if the sauce mixes slightly into the filling.
To the shallow soak bowl, add the water, remaining 2 tablespoons (25g) granulated sugar, and remaining 1 tablespoon espresso powder. Whisk to combine and dissolve the sugar.
Repeat the soaking process (step 8), quickly dipping both sides of the remaining ladyfingers in the soak, and arranging them on top of the filling and caramel.
Drizzle another scant 1/2 cup (110g) of caramel sauce evenly over top, then use a small offset spatula or the back of a spoon to spread it into a thin layer. (You’ll be left with a couple of tablespoons of caramel sauce; set it aside in a covered container in the refrigerator to use when serving.) Dollop the remaining mascarpone filling over top, then spread evenly.
Loosely cover the tiramisu and chill in the refrigerator for at least 2 hours or preferably overnight.
When ready to serve, decoratively drizzle the reserved caramel sauce over the surface of the tiramisu. (Don’t be tempted to warm up the caramel sauce to make it thinner, as warm sauce will melt the top layer of mascarpone filling.) Slice and serve.
Storage information: Store leftover Caramel Macchiato Tiramisu, covered, in the refrigerator for several days.
