In a heatproof bowl, whisk together the egg yolks, sugar, cornstarch, and salt until thick and pale in color.
In a medium saucepan, heat the milk to a gentle boil. Gradually add about 1 cup of the hot milk into the egg yolk mixture to temper the yolks, whisking constantly so they don't curdle, then whisk in the remainder of the milk in a steady stream.
Set the bowl over a saucepan of simmering water (do not let the bottom of the bowl touch the water) and cook, whisking constantly, until thickened, 5 to 7 minutes. Remove from the heat and whisk in the vanilla. Let the custard sit for 2 to 3 minutes to cool slightly, then gently whisk in the butter until it is melted and the custard is smooth and silky.
Place a piece of plastic wrap directly on top of the custard so that a skin does not form and let cool for 30 minutes, or until it reaches room temperature.
Spoon the cooled filling into the prepared piecrust. Cover with plastic wrap and place it in the refrigerator to chill for at least 3 hours, or overnight.
To finish the tart, in a small saucepan, heat the preserves over low heat, stirring, until liquified and smooth. Set aside to cool.
Pile the strawberries on top of the custard filling. Brush the preserves all over the berries. Refrigerate until ready to serve, or for up to 3 days. Serve with whipped cream.