Prepare the Mango Jam: Heat 2 cups of frozen mangoes in a saucepan over medium heat until softened, about 5-7 minutes. Mash with 1/4 cup of white sugar until well combined, then cool in the fridge for at least 30 minutes.

Cook the Sticky Rice: Soak 1 cup of sweet rice in boiling water for 30 minutes. Steam in a lined steamer for 20-25 minutes, then stir in 1/2 cup coconut milk and 1/4 cup sweetened condensed milk.

Make the Cookie Dough: Cream together 1/2 cup unsalted butter, 1/4 cup light brown sugar, and 1/4 cup white sugar for 2-3 minutes. Beat in one egg yolk, a thick egg white, and 1 teaspoon vanilla extract. Gradually mix in 1/2 teaspoon baking soda, 1/4 teaspoon salt, and 1 1/2 cups all-purpose flour.

Chill the Dough: Cover the dough with plastic wrap and refrigerate for at least 30 minutes.

Assemble and Bake: Preheat oven to 350°F (175°C). Fold in the cooled sticky rice and 1/4 cup mango jam into the dough. Portion out the dough onto a lined baking tray, topping with flaky sea salt or black sesame seeds if desired.

Bake the Cookies: Bake for 13-15 minutes until edges are lightly golden, and allow them to cool on the tray for 10 minutes before transferring to a wire rack.