In a small bowl, combine the sugar and matcha. Add a little bit of hot water and stir vigorously until well combined with no lump. Add the remaining hot water and stir to combine.

Let the mixture cool completely. Stir in the rum.

In a large bowl using a handheld mixer or stand mixer fitted with a whisk attachment, beat heavy whipping cream, confectioners' sugar, and vanilla extract on high speed until stiff peaks form.

Add mascarpone cream and beat on low speed until well incorporated. Refrigerate the mascarpone until using it.

Line the side of a 6x3 inch/15x7.5 cm cake ring or springform pan with baking paper or transparent cake collar. If using a cake ring, place a cake board or a flat surface on the bottom.

Trim the ladyfingers to fit the pan (I use 8 ladyfingers for each layer).

One at a time, gently dip both sides of ladyfingers into the matcha simple syrup, then lay them on a single layer on the bottom of the pan. Spread one-third mascarpone cream over the top. Repeat the process for the second and third layers.

Smooth the cream on top using a bench scrapper or offset spatula.

Chill for at least 6 hours or overnight to set.

Remove the mold and baking paper/cake side membrane. Smooth the sides of the cake with a bench scrapper or offset spatula.

Pipe whipped cream on top to decorate.

Dust the top with matcha powder generously right before serving.