In a stainless steel saucepan, heat the milk with the vanilla bean and tarragon to a simmer. Let sit 5 minutes to infuse.
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.
Preheat a grill or indoor grill pan. Sprinkle the pineapple with powdered sugar and grill, turning once, until well marked. Cool slightly.
To serve, place 1 grilled pineapple spear in the glass, sticking out, and a lime wedge to squeeze over the whole dessert.