Preheat oven to 325 degrees F (165 degrees C). Grease insides of four 6-ounce ramekins; lightly dust with 1 teaspoon sugar.
Heat milk in a small saucepan over medium heat until it begins to bubble.
Place coconut milk in the top of a double boiler over simmering water. Heat until edges start to bubble, about 5 minutes. Whisk in Thai tea powder. Cook, scraping the sides and bottom of the bowl, until dissolved, about 5 minutes.
Strain coconut milk into a separate bowl through a fine sieve or cheesecloth; whisk in sucralose sweetener, vanilla extract, and coconut extract.
Place egg whites and cream of tartar in a glass, metal, or ceramic bowl. Beat with an electric mixer until stiff peaks form.
Whisk egg yolks and 1/3 cup sugar together in a separate bowl. Add coconut milk mixture, flour, and salt. Whisk until batter is evenly mixed. Fold in egg whites until smooth. Ladle the batter into the prepared ramekins. Set ramekins on a baking pan. Pour boiling water into the pan to reach halfway up the ramekins.
Bake in the preheated oven until tops are lightly browned and cakes pull way from the edges of the ramekins, 40 to 50 minutes. Let cool until set, at least 10 minutes. Loosen edges of cakes with a sharp knife; invert onto small plates.
