Whisk cornstarch, white rice flour, baking powder, and salt in a medium bowl. Set aside.
In a large bowl, beat the softened butter with the sugar until creamy. Mix in the vanilla extract and lemon zest.
Add the egg and egg yolks one at a time, mixing well after each addition.
Gradually add the dry ingredients and mix just until a soft dough forms. Do not knead or overwork the dough.
Cover the dough and refrigerate for at least 30 minutes.
Preheat the oven to 340°F (170°C). Line a baking sheet with parchment paper or a silicone mat.
Lightly dust your work surface with cornstarch. Roll the dough to about 1 cm thick and cut out rounds using a 1- to 2-inch cookie cutter. Gather the scraps, gently bring them together, and continue cutting.
Place the cookies on the prepared baking sheet, leaving a little space between them. Bake for 10–12 minutes. The cookies should remain pale.
Let the cookies cool completely before removing and filling them.
Spread with dulce de leche on one side of one cookie. Top with another cookie and gently press together. Roll the edges in shredded coconut.