Let's cook.
In a microwave-safe bowl, combine 1 cup glutinous rice flour, 1/4 cup granulated sugar, 1/4 teaspoon matcha powder, and 1/4 teaspoon black sesame powder. Whisk well to combine.
Gradually add 3/4 cup water to the dry ingredients, stirring until a smooth, lump-free batter forms.
Cover the bowl loosely with plastic wrap and microwave on high for 2 minutes. Carefully remove, stir the mixture, and re-cover.
Microwave for another 1 to 1.5 minutes, or until the mochi dough is translucent and sticky. It should be very pliable.
Dust a clean work surface generously with 1 tablespoon cornstarch. Carefully transfer the hot mochi dough onto the cornstarch.
Using a cornstarch-dusted rolling pin, gently roll the mochi dough into a rectangle about 1/4 inch thick. Cut the dough into 8 equal pieces.
In a small bowl, mix the remaining 1/4 teaspoon matcha powder and 1/4 teaspoon black sesame powder. Dust each mochi piece with this mixture for extra flavor and color.
Serve immediately or allow to cool slightly before enjoying. The mochi are best eaten fresh.
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