Step 1

Place a rack in middle of oven; preheat to 350°. Generously butter an 8"- or 9"-diameter cake pan. Whisk mochiko, baking powder, salt, and 2 Tbsp. matcha in a large bowl just to combine. Vigorously whisk eggs and melted butter in a medium bowl until pale and emulsified, about 30 seconds. Add sweetened condensed milk, vanilla, and 1 cup cream and whisk until mixture is smooth. Scrape into dry ingredients and whisk vigorously until smooth and very thick. Scrape batter into prepared pan and smooth surface.

Step 2

Bake cake until it starts to pull away from sides of pan, top and edges are golden, and a tester inserted into the center comes out clean, 35–45 minutes (cake in the smaller pan will take longer to bake than one in the larger pan). Transfer pan to a wire rack and let cake cool 5 minutes. Turn cake out onto rack; let cool completely.

Step 3

Place chocolate in a small heatproof bowl. Bring remaining ⅓ cup cream to a boil in a small saucepan. Immediately pour over chocolate; let sit 15 seconds. Stir until ganache is smooth.

Step 4

Evenly pour ganache over top of cake. Using a small offset spatula or spoon, spread ganache to edges of cake (drips down the sides are encouraged!). Let sit until ganache is set, about 2 hours. (If you are in a hurry, you can chill cake until glaze is set, about 30 minutes.)

Step 5

Just before serving, dust top of cake with more matcha with a fine-mesh sieve.Do ahead: Cake and glaze can be made 2 days ahead. Store cake tightly wrapped in several layers of plastic at room temperature. Let glaze cool, then chill in an airtight container. Gently reheat glaze in the microwave or a double boiler over a pan of simmering water (do not let bowl touch water) until just melted before using.