Preheat your oven to 350°F (175°C) and grease a 9-cup muffin tin with unsalted butter.

In a medium saucepan, combine full-fat coconut milk, bring to a gentle boil, add Thai tea leaves and steep for about 7 minutes.

In a medium mixing bowl, whisk together mochiko rice flour, dark brown sugar, baking powder, and salt.

Strain the coconut milk to remove tea leaves, transfer to a large bowl and add egg, vanilla extract, and melted butter; mix thoroughly.

Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.

Pour the batter into the prepared muffin tin, filling each cup about 1/4 inch from the top.

Sprinkle sesame seeds over the tops and bake for 55 to 65 minutes until golden brown.

Allow to cool for about 10 minutes, then transfer to a wire rack for further cooling before serving.