In a medium saucepan over medium heat, cook 200g of frozen mangoes until softened, about 5-7 minutes. Mash the mangoes, then stir in 1 tablespoon of white sugar until dissolved. Remove from heat and let it cool slightly. Refrigerate the jam until it thickens, approximately 30 minutes to 1 hour.

Rinse 0.25 cup of sweet rice under cold water until the water runs clear. Soak the rice in water for 4 hours or overnight for the best texture. Drain and place the rice in a steamer lined with cheesecloth or parchment paper. Steam for about 20 minutes until slightly underdone. Transfer the rice to a bowl and gently mix in 0.33 cup coconut milk and 3 tablespoons sweetened condensed milk.

In a large mixing bowl, cream together 113g unsalted butter, 100g light brown sugar, and 50g white sugar until the mixture is light and fluffy, about 3-5 minutes. Add in one large egg yolk, a thick egg white, and 0.5 teaspoon vanilla extract, mixing until well combined.

Slowly incorporate the dry ingredients – 180g all-purpose flour, 0.5 teaspoon baking soda, and 0.25 teaspoon salt – mixing just until combined to maintain chewiness.

Take the cookie dough and wrap it in plastic wrap. Refrigerate the dough for at least 30 minutes.

Preheat the oven to 350°F (175°C) and line a baking tray with parchment paper. Once the dough has chilled, remove it from the refrigerator. Gently fold small amounts of sticky rice and mango jam into the dough.

Scoop tablespoons of dough onto the prepared tray, pressing in optional toppings like flaky sea salt or black sesame seeds.

Bake the cookies in the preheated oven for 13-15 minutes, or until they're golden. Let the cookies cool on the tray for about 10 minutes before transferring them to a wire rack to cool completely.