Bring the cream to a simmer in a medium saucepan over medium heat. Meanwhile, whisk the egg yolks, sugar, and salt together in a medium bowl, then whisk in the cornstarch until pale yellow and thick, about 30 seconds.

Slowly whisk the hot cream into the egg mixture to temper, then return the mixture to the saucepan and cook over medium heat, whisking constantly, until thick and glossy, about 1½ minutes. Off the heat, whisk in the butter and vanilla. Transfer the mixture to a small bowl, press plastic wrap directly on the surface, and refrigerate until chilled and set, about 2 hours.

Microwave all of the ingredients together, whisking often, until melted and smooth, 1 to 2 minutes. Let the glaze cool, uncovered, until it is thickened but pourable, about 20 minutes.

Prepare cake mix according to directions on the box. Spoon heaping tablespoonfuls into the wells of greased mini muffin pans. Bake for about 8 minutes or until a toothpick inserted into the center comes out clean. Invert on a baking rack and let cool completely.

With a serrated knife, cut the curved dome off the top of each muffin.

Spoon chilled pastry cream into a pastry bag or ziplock bag and snip of the end. Squeeze a dollop of cream onto ½ of the trimmed cakes. Place the remaining cakes on top of the cream to create a sandwich.

Transfer glaze into a squeeze bottle or use a spoon to top each sandwich with the glaze.