In a large bowl, mix together the mascarpone and vanilla extract.

In another large bowl, use an electric mixer or stand mixer to beat the cream and sugar on medium speed for 5 to 8 minutes, or until soft to medium peaks form. Gradually and gently fold the whipped cream into the mascarpone, mixing until just combined.

Assemble the tiramisu. In a medium bowl, mix together the espresso and rum, if using. Dip half the ladyfingers into the espresso one at a time, leaving each one in the liquid just long enough to coat it on all sides. If it stays in the liquid too long, it will get soggy. Arrange the dipped ladyfingers evenly at the bottom of a 9x9-inch or similar baking dish, cutting them if necessary to fully cover the bottom.

Spread half the mascarpone mixture evenly on top. Dip the remaining ladyfingers into the espresso one at a time, placing them in an even layer on top of the mascarpone. Spread the remaining mascarpone mixture evenly on top.

Cover and chill in the refrigerator for at least 8 hours or overnight. When ready to serve, place the cocoa powder in a fine mesh strainer and dust over the top of the tiramisu. Slice and serve.