Make the Shortbread Crust: Pulse the flour, sugar and salt together in the bowl of a food processor. Scatter the pieces of butter over the dry ingredients and pulse until the mixture looks like coarse sand and there are no pieces of butter larger than the size of a pea (about 5 to 10 pulses) Add the egg a little at a time, pulsing after each addition. Once all of the egg has been added, process until the dough forms large clumps (about 10 seconds).
Turn the dough into a 13¾x4¼-inch rectangular tart pan and press into an even layer all over the bottom and up the sides of the tart pan. Run a rolling pin over the top of the tart pan to remove the excess dough. Place the dough-lined pan in the freezer for at least 30 minutes.
Preheat oven to 375 degrees F. Pierce the dough all over with a fork, then cover the crust with aluminum foil, pressing the foil tightly against the crust and covering the sides. Place the tart pan on a baking sheet and bake for 20 to 25 minutes, or until lightly browned. Carefully remove the foil. If the tart has puffed up at all, press it down gently with the back of a spoon. Return to the oven and bake for an additional 10 to 15 minutes, or until golden brown. Remove from the oven and place the tart pan on a wire rack and allow to cool completely.
Make the Caramel Filling: In a small saucepan over low heat, combine the heavy cream and sweetened condensed milk, stirring to combine until the sweetened condensed milk is melted and incorporated into the cream. Remove from heat and cover to keep warm.
Place the sugar, golden syrup, and water in a medium saucepan and warm over low heat, stirring occasionally, until the sugar melts. Increase the heat to medium and cook, not stirring, until the mixture reaches 250 degrees F on a candy or instant-read thermometer. Remove from the heat and stir in the butter and the cream mixture (pour the cream mixture carefully, as the caramel will bubble up). Return to medium heat and cook, stirring constantly, until it reaches 240 degrees F. Pour the caramel into the shortbread crust and allow to cool and set completely, at least 30 minutes.
Make the Milk Chocolate Ganache: Place the chopped chocolate in a small bowl; set aside. Place the heavy cream in a small saucepan and bring to a rapid simmer over medium heat, then pour over the chocolate. Let sit for 2 minutes, then stir until completely smooth. Pour the ganache over the caramel layer, spreading it into an even layer with an offset spatula if necessary. Refrigerate the tart for at least 30 minutes to allow the ganache to set. Allow to sit at room temperature for 20 minutes before serving. Remove the tart from the pan, slice and serve. Leftovers can be stored in an airtight container, or tightly wrapped in plastic wrap, in the refrigerator for up to 5 days.