In a shallow bowl, combine cooled espresso with brandy or rum extract if using. Set aside.
In a large mixing bowl, beat the heavy cream with an electric mixer until stiff peaks form. Transfer to another bowl and refrigerate.
In the same bowl (no need to wash), beat mascarpone cheese, powdered sweetener, and vanilla extract until smooth and creamy, about 2 minutes.
Gently fold the whipped cream into the mascarpone mixture until fully combined and fluffy.
Quickly dip each ladyfinger into the espresso mixture (just 1-2 seconds per side) and break into pieces to fit the bottom of 8 dessert cups or small glasses.
Spoon a layer of mascarpone cream over the ladyfingers, then repeat with another layer of dipped ladyfingers and cream.
Cover and refrigerate for at least 4 hours or overnight to allow flavors to meld.
Just before serving, dust generously with unsweetened cocoa powder using a fine-mesh sieve.