In a large saucepan, heat ½ cup milk and 1 tablespoon sugar over medium heat, whisking constantly, until steaming. Remove from heat.
In a medium bowl, whisk together 2 egg yolks, cornstarch, flour, and 1 tablespoon sugar. Add about half of warm milk mixture to egg mixture, whisking constantly. Add egg mixture to remaining hot milk mixture, whisking to combine. Cook over medium heat, stirring constantly, until mixture is thickened, about 3 minutes. Strain mixture through a fine-mesh sieve, discarding solids. Cover with a piece of plastic wrap, pressing wrap directly onto surface of cream. Refrigerate for at least 1 hour.
In the top of a double boiler, combine chocolate, butter, and vanilla. Cook over simmering water until melted and smooth. Remove from heat; whisk in almond flour and remaining 3 tablespoons sugar. Whisk in egg and 1 egg yolk. Cover and refrigerate until cool, at least 30 minutes.
Line a baking sheet with parchment paper.
Fold chilled pastry cream into chilled chocolate almond cream. On a lightly floured surface, cut 4 (4-inch) circles from each puff pastry sheet. Place 8 rounds on prepared pan. Using a fork, lightly pierce rounds all over. Brush edges of one round with water. Place 2 tablespoons chocolate almond cream mixture in center, leaving a ¾-inch border around edges. Top with another round, and press with fingertips to seal edges. Using a sharp paring knife, cut a design on top of galette, and notch edges. Repeat with remaining rounds and chocolate almond cream mixture.
In a small bowl, whisk together remaining 1 egg yolk and remaining 1 teaspoon milk. Brush egg wash onto dough. Refrigerate for 30 minutes.
Preheat oven to 400°.
Bake for 15 minutes. Reduce oven temperature to 375°, and bake until deep golden brown, about 15 minutes more. Let cool completely on a wire rack.
Recipe by Louisiana Cookin' at https://louisianacookin.com/mini-chocolate-galettes-des-rois/