**Prepare the Rhubarb Layer:** In a medium saucepan, combine the chopped rhubarb, 1/2 cup granulated sugar, and 1/4 cup water. Bring to a gentle boil over medium heat, then reduce heat to low and simmer for 10-15 minutes, or until the rhubarb is tender and breaks down into a compote-like consistency. Stir occasionally. Remove from heat and let cool completely.

**Prepare the Vanilla Pudding Layer:** In a separate medium saucepan, whisk together 1/2 cup granulated sugar, cornstarch, and salt. Gradually whisk in the whole milk until smooth. Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil. Continue to cook, stirring, for 1 minute.

In a small bowl, lightly beat the egg yolks. Temper the egg yolks by gradually whisking about 1/2 cup of the hot milk mixture into the yolks. Slowly pour the tempered egg yolks back into the saucepan with the remaining milk mixture, whisking constantly. Cook for another 1-2 minutes, stirring, until the pudding is very thick. Do not boil.

Remove the pudding from the heat and stir in the unsalted butter and vanilla extract until melted and combined. Pour the pudding into a clean bowl, cover the surface directly with plastic wrap (to prevent a skin from forming), and refrigerate until completely chilled, at least 2 hours.

**Prepare the Cookie Crumble Topping:** In a medium bowl, combine the crushed shortbread cookies and melted butter. Stir until the crumbles are evenly coated.

**Assemble the Dessert:** In individual serving dishes or a large trifle bowl, layer the cooled rhubarb compote, followed by the chilled vanilla pudding, and then a generous layer of the cookie crumble. Repeat layers if using a larger dish. For best results, chill for at least 30 minutes before serving to allow the flavors to meld.

Garnish with fresh mint leaves or a dollop of whipped cream, if desired, before serving.