Add milk to medium saucepan over medium-high heat. Add contents of natilla box to milk. Using whisk, stir until smooth. Bring milk mixture to low simmer, whisking constantly until smooth and thick. Reduce heat to lowest setting. Add chocolate; whisking until fully melted and incorporated into custard. Remove from heat.
To build parfaits, line up 4 8-oz. parfait glasses or wine glasses. Spoon ¼ cup chocolate pudding into each glass. Top pudding with 2 Maria cookies and 5 strawberry halves. Repeat layering once more. Then, divide remaining pudding among each glass (about 2 tbsp. each); cover glasses with plastic wrap. Chill until pudding is firm and cookies soften, at least 2 hours, or up to 48 hours. Just before serving, add dollop of whipped cream and decorate with remaining strawberry halves.