Step 1

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

Step 2

, In a small (1 1/2-quart) saucepan, bring the milk to a boil. Add the hot milk to the egg mixture gradually, whisking continually to make everything smooth.

Step 3

, 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.

Step 4

, 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.

Step 5

, To assemble the tart: 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.

Step 6

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

Step 7

, To make the glaze: If you're using jam or preserves, warm them gently and thin with a little water if necessary to loosen them up. Strain or scoop out and discard any solids, then brush the remaining glaze over the berries. If you're using jelly, simply warm it and brush it atop the berries.

Step 8

, Slice the tart into wedges and serve with whipped cream, if desired.

Step 9

, Storage information: Store any leftover tart, covered, in the refrigerator for several days. We don't advise freezing this tart.