Beat the butter and icing sugar together until smooth.
Push the hard-boiled egg yolk through a sieve and stir the raw egg yolk and vanilla into it. Add this to the butter mixture and stir until blended.
Add the flour and salt to the butter mixture and stir until blended. Shape the dough into a disc (it will be very soft), wrap in plastic and chill until firm, about 2 hours.
On a lightly floured work surface, gently knead the dough just a little soften, then roll it out to a circle about 12 inches across and 1/4-inch thick. Carefully lift this and line a 9-inch removable-bottom fluted tart pan, press it into the bottom and sides and trim away any excess dough. Chill the tart shell for 30 minutes.
Preheat the oven to 180°C
Place the chilled tart shell onto a baking tray and dock the bottom of the pastry with a fork. Bake the tart shell for about 20 minutes, until the edges just begin to brown. Cool the tart shell to room temperature.
For the custard filling, bring the all but 2 Tbsp of the milk, the vanilla and the orange zest to just below a simmer over medium heat. Whisk the egg yolks with the sugar, cornstarch and remaining 2 Tbsp of milk in a small bowl. Slowly pour the hot milk into the egg mixture while whisking and then pour the entire mixture back into the pot. Whisk this over medium heat until the custard just comes to a boil, thickens and becomes glossy, about 5 minutes. Have a bowl with butter in it and a strainer on top ready for when the custard has thickened. Remove the pot from the heat and strain the custard into a clean bowl and stir until the butter is melted in. Add the liqueur (if using) and then cover with plastic wrap so that the wrap is directly on the surface of the custard. Cool this to room temperature, then chill for at least 2 hours.
To assemble the tart, whisk the custard to soften it and then spread it evenly over the bottom of the cooled tart shell (leave the tart shell in the pan). Arrange the slice peaches and raspberries over the custard.
Melt the apple jelly over medium-low heat, whisking until smooth. Brush the peaches and raspberries with the melted apple jelly and chill until ready to serve. Carefully remove the tart from the pan and place on a serving platter before slicing.
The tart is best served the day it is made, but can be stored chilled for up to a day.
Note: if your peaches are firmer, then blanch in hot water followed by a shock into a bowl of ice water and then peel.