Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal later.
In a large mixing bowl, whisk together the sweet rice flour, sugar, baking powder, and salt until evenly combined with no lumps.
In a separate medium bowl, whisk the coconut milk, eggs, melted coconut oil, lemon juice, lemon zest, and vanilla extract until smooth and well blended.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. The batter will be thin and pourable, similar to pancake batter.
Fold in the shredded coconut until evenly distributed throughout the batter.
Pour the batter into the prepared pan and spread it into an even layer. Tap the pan gently on the counter a few times to release any air bubbles.
Bake for 35-40 minutes, until the edges are golden brown and set, but the center still looks slightly jiggly and glossy. A toothpick inserted in the center should come out with a few moist crumbs.
Let the mochi cool completely in the pan for at least 1 hour. The texture will firm up and become properly chewy as it cools.
Use the parchment overhang to lift the mochi out of the pan, then cut into 24 squares with a sharp knife. Dust with powdered sugar before serving if desired.
