Pulse shortbread cookies in a food processor until you have fine, uniform crumbs (about 2½ cups). Stir together with melted butter and salt until the mixture resembles wet sand and holds when pressed.
Press the crumb mixture firmly into the bottom and up the sides of a 9-inch tart pan with removable bottom. Refrigerate for at least 20 minutes while you prepare the filling.
Whisk together 4 whole eggs and 4 egg yolks in a medium bowl until well blended and slightly frothy, then set aside at room temperature.
Melt 12 tablespoons butter in a medium saucepan over medium-low heat, then whisk in sugar, lemon zest, lemon juice, cornstarch, and salt. Once hot and steaming, slowly whisk in the egg mixture and continue whisking constantly for 6–8 minutes until thickened to a pudding-like consistency.
Pour the curd through a fine-mesh strainer into a heatproof bowl, pressing with a spatula. Transfer 1 cup of strained curd to a separate bowl and stir in the seedless strawberry jam until smooth and evenly pink.
Alternately pour dollops of plain lemon curd and strawberry-lemon curd into the chilled crust until all curd is used, then drag a skewer or chopstick through the surface in swirling motions to create a marbled pattern.
Carefully transfer the tart to the refrigerator (keeping it level) and chill for at least 2 hours, or ideally overnight, until the curd is completely set and no longer jiggly.
Remove the outer ring by setting the pan on a large can and pressing the ring down, then slide a thin spatula under the crust to transfer to a serving platter. Slice with a clean knife, wiping between cuts, and serve with whipped cream and fresh strawberries.