In a medium saucepan over medium heat, combine frozen or fresh mangoes. Mash until soft, stirring in white sugar until dissolved. Simmer for 5-7 minutes until thickened, then cool in the fridge.

Soak sweet rice in boiling water for 30 minutes, then steam for 20-25 minutes until slightly underdone. Mix cooked rice with coconut milk and sweetened condensed milk, reserving a tablespoon.

Cream room temperature unsalted butter with light brown sugar and white sugar for 2-3 minutes. Beat in reserved coconut cream, egg yolk, and vanilla extract until combined. Gradually mix in baking soda, salt, and all-purpose flour until just combined.

Cover the cookie dough with plastic wrap and refrigerate for at least 30 minutes. Preheat oven to 350°F (175°C) and prepare a baking tray.

Swirl in reserved sticky rice and cooled mango jam into the dough. Scoop portions onto prepared tray, leaving space in between. Bake for 13-15 minutes until edges are golden, then cool on the tray for 10 minutes.

Transfer cookies to a wire rack to cool completely. Sprinkle with flaky sea salt or top with black sesame seeds if desired.