Have ready a 10-inch tart pan with fluted sides and a removable bottom.
Melt 2 1/2 cups of the chocolate and 2 tablespoons of the shortening in a small microwave-safe bowl on high power, in 20 second bursts, stirring after each one, until smooth, about one minute. Alternatively, place the bowl over a pot of simmering water, over medium to medium-high heat, stirring occasionally, until melted.
Spread the melted chocolate on the bottom of the tart pan and up the sides with the back of a spoon or with a small offset spatula. Place the tart pan in the freezer until the chocolate is hard, about 25 minutes.
Combine the peanut butter, butter, vanilla, salt and sugars in a medium-sized microwave-safe bowl and heat on high power, for 60 to 90 seconds, until the butter melts. Alternatively, place the bowl over a pot of simmering water, over medium to medium-high heat, stirring occasionally until the butter is melted. Stir to combine with a rubber spatula.
Transfer the peanut butter mixture to the chocolate-coated tart pan and spread with an offset spatula over the chocolate. Place back in the freezer while you melt the remaining chocolate.
Melt the remaining chocolate and shortening as you did the first -half, and pour over the peanut butter filling in two stages, smoothing with an offset spatula.
Sprinkle with flaky sea salt, if you so desire, and place in the freezer to set up, about 45 minutes. This time in the freezer will facilitate the removal of the tart bottom.
When ready to serve, remove the sides of the tart pan and then the bottom, using a small paring knife, to dislodge it, if necessary. Place on a serving platter and let sit for about 20 minutes or so, depending on how warm your kitchen is, to soften slightly. Slice with a large chef’s knife, dipped in hot water and wiped dry, and serve with glasses of milk.
The giant cup will keep wrapped in plastic wrap in the refrigerator for up to a week.