In a medium bowl, whisk together the egg, egg yolks, sugar, cornstarch, and salt for about 2 minutes until the mixture is smooth and lightened.

Heat the milk and butter in a large saucepan over medium heat until just simmering, stirring occasionally. Remove from heat.

Gradually pour one-third of the hot milk into the egg mixture while constantly whisking to prevent curdling.

Return the mixture to the saucepan with the remaining milk and cook over medium heat, whisking continuously, until thickened. Reduce heat to medium-low and let it boil gently for 1 minute.

Remove from heat, stir in vanilla, and transfer to a shallow bowl. Cover with plastic wrap pressed directly onto the surface to prevent skin from forming. Let cool, then refrigerate for 2 hours or up to 3 days.

In a stand mixer fitted with a paddle attachment (or using a hand mixer), beat the egg and sugar on medium speed for 30 seconds until blended.

Add butter and mix on high speed for 2 minutes until thick and creamy, resembling buttercream.

Mix in the mayonnaise and salt. Add flour in thirds, mixing between additions, until the dough is smooth and slightly sticky.

Roll the dough out on a floured surface to ⅛-inch thickness. Cut 12 rounds using a 3-inch cookie cutter or a mason jar ring. Re-roll scraps as needed.

Press each dough round into the wells of a nonstick cupcake tin, shaping it up the sides. Prick the bottoms with a fork and refrigerate for 30 minutes.

Preheat the oven to 350°F. Bake the tartlet shells for 10 to 12 minutes until golden, then cool completely. Store in an airtight container for up to 2 days.

Warm the apricot jam with 1 tablespoon of water in the microwave or on the stovetop until smooth.

Fill each tart shell with about 2 tablespoons of chilled pastry cream.

Top with fresh berries, then brush lightly with the jam glaze. Serve chilled.