Combine the water, sugar, and matcha powder in a small saucepan. Bring to a simmer, stirring occasionally to dissolve sugar. Remove from heat and set aside to cool completely.

In a stand mixer fitted with the whisk attachment, whip the heavy cream until stiff peaks form. Transfer whipped cream to a separate bowl, return bowl to the mixer, and replace whisk attachment with paddle attachment.

To the mixer, add the mascarpone cheese,  sugar, and vanilla. Beat on high until combined. Gently fold in the whipped cream until fluffy and combined.

One at a time, quickly dip the ladyfingers in the matcha syrup until soaked but not soggy; arrange in an even layer along the bottom of an 8-inch square dish. (Line the ladyfingers up in two straight rows of six.*) Spread with half of the the mascarpone filling. Repeat with remaining ladyfingers, syrup, and filling.

Cover with plastic wrap and refrigerate for at least 6 hours before serving. If desired, pipe dollops of whipped cream evenly over the entire surface of the Tiramisu. Just before serving, place matcha powder in a fine mesh sieve and shake a light dusting over the surface. Cut into squares and serve.