This combines tropical flavors of pineapple, coconut, and lime in a simple, cooling custard. The tarragon gives a twist of licorice flavor. Serving it with the pineapple hot off the grill gives a great temperature contrast between the hot fruit and the cold custard.
In a separate bowl whisk together the egg yolks and sugar until light and fluffy. Whisk in the cornstarch; then drizzle in the milk mixture while whisking. Pass through a fine-meshed sieve back into the pot, to remove the tarragon and vanilla bean. Bring to a boil, whisking constantly. Remove from the heat, stir in the coconut and butter to melt it. Place in a container over an ice bath and fold it to cool.
Once it's cold, fold in the whipped cream and spoon into dessert cups or stemmed glassware and chill.
The custard keeps for up to 3 days, chilled and covered.
To serve, place 1 grilled pineapple spear in the glass, sticking out, and a lime wedge to squeeze over the whole dessert.