Place the ¼ of a cup of Tajín chile powder on a small flat plate.
Take one quarter of a lime and rub it along the edges of the two glasses you will be serving your margarita in. Immediately place the rims of the glasses on the plate with the chile and twist to coat.
Combine the tequila, Cointreau, lime juice, pineapple juice, chopped pineapple, 1 tsp. of Tajín chile powder, and 1 cup of ice. Stir to blend well.
Divide the remaining one cup of ice between the two glasses and pour margarita mixture over.
You can garnish with a wedge of pineapple dipped in chile powder