Prepare the Rhubarb-Strawberry Topping: In a medium saucepan, combine the chopped rhubarb, strawberries, 1/2 cup sugar, and 2 tablespoons cornstarch. Stir well to combine and coat the fruit.

Cook the topping over medium heat, stirring occasionally, until the rhubarb has softened and the mixture has thickened into a compote. This will take about 10-15 minutes. Remove from heat and set aside to cool slightly.

Prepare the Vanilla Pudding Base: In a separate medium saucepan, whisk together the milk, 3/4 cup sugar, and 1/4 cup cornstarch until smooth.

In a small bowl, whisk the egg yolks. Gradually temper the egg yolks by slowly whisking about 1/2 cup of the milk mixture into them. Then, pour the tempered egg yolk mixture back into the saucepan with the rest of the milk mixture.

Cook the pudding base over medium heat, stirring constantly, until it thickens considerably and just begins to bubble. Be sure to stir from the bottom to prevent scorching. This will take about 8-10 minutes.

Remove the pudding from the heat. Stir in the butter pieces until melted and incorporated. Stir in the vanilla extract.

Assemble the dessert: Spoon the vanilla pudding into individual serving dishes or a larger trifle bowl. Top generously with the cooked strawberry-rhubarb mixture.

Chill the dessert in the refrigerator for at least 1 hour before serving to allow it to set completely. Garnish as desired.