In a medium heatproof bowl, whisk together the sugar, flour, cornstarch, and salt. Add the egg, whisking until the mixture is smooth

In a small saucepan, bring the milk to a boil. Add the hot milk to the egg mixture very slowly so as not to scramble the egg, whisking continually to make everything smooth.

Pour the hot milk/egg mixture back into the saucepan, return to the heat, and bring to a boil, whisking continually. The mixture will thicken quickly and whisking vigorously will keep it from getting lumpy.

Once you see the filling come to a boil across its entire surface, cook for another minute, then remove from the heat. Stir in the butter and extract.

Pour the filling into the baked tart shell. Place a piece of plastic wrap directly on the surface to prevent a skin from forming and refrigerate.

When you’re ready to serve the tart, remove the wrap from the filling and top the tart with the fruit in alternating rows; or simply arrange the fruit artfully atop the filling.

Warm the jam and thin with a little water if necessary. Strain or scoop out and discard any solids, then brush the resulting glaze over the berries.