Prepare the Mango Jam: Heat the frozen mangoes in a small pot over medium heat until softened. Mash them gently, add the sugar, and stir until reduced to a jam-like consistency. Allow the jam to cool in the fridge.

Cook the Sticky Rice: Soak the sweet rice in boiling water for 20 minutes, then steam it until slightly underdone. In a separate pot, heat the coconut milk and sweetened condensed milk, and mix thoroughly with the cooked rice until all grains are coated and plump. Reserve a tablespoon for the cookie dough.

Make the Cookie Dough: In a large mixing bowl, cream the softened butter with both sugars until fluffy and light. Incorporate the egg yolk, thick egg white, vanilla extract, and reserved coconut cream. Mix in the baking soda and salt, then gradually add the flour, mixing until just combined. Chill the dough in the fridge for at least 30 minutes.

Assemble and Bake: Preheat your oven to 350°F (175°C) and line a baking tray with parchment paper. Dollop small amounts of sticky rice and mango jam onto the cookie dough, swirling gently. Use a cookie scoop to portion the dough, gently mixing in extra mango and rice as needed. Press any desired toppings like black sesame seeds onto the dough balls and leave space for spreading. Bake for 13-15 minutes until the edges are golden brown and the centers look slightly puffed. Let the cookies cool on the tray for 10 minutes before transferring them to a wire rack.