In a large mixing bowl, use an electric hand mixer to beat softened butter until it becomes creamy. Add the powdered sugar and mix on low speed until fluffy.
Add the egg and mix until smooth. Add the flour and salt and mix again on low speed until the dough starts to come together and not too many floury patches remain.
Turn the dough out onto a sheet of plastic wrap. Use your hands to pack the dough together. Split the dough into halves and press them each into a flattened disc shape. Wrap the dough with plastic wrap and let it chill in the fridge for at least 4 hours or overnight. You can freeze one of the dough discs to use another time.
Preheat the oven to 350°F. When you're ready to roll the dough out, let one disc sit at room temperature for 15-30 minutes to soften slightly. On a floured surface, roll the dough out to about ⅛" thick, lifting and rotating the dough occasionally to prevent it from sticking to the counter.
Transfer the dough to a 9" fluted tart pan. Press the dough into the edges and sides of the pan. Don't worry if the dough breaks - you can easily patch up any bald spots by pressing small pieces of dough on top of them.
Use the rolling pin to roll across the top of the pan to cut the excess dough off the edges of the pan. Use a fork to dock the bottom of the tart shell (poke holes across the bottom of the shell).
Freeze the tart pan for about 30 minutes until it becomes firm. This helps prevent shrinkage during baking. Gently press a sheet of foil into the tart shell and fill it with pie weights or uncooked rice.
Bake the tart shell for about 15 minutes. Carefully remove the foil and pie weights and bake for another 15-20 minutes until the edges are golden brown. Let the tart shell cool on a wire rack before releasing it from the pan.
Custard Filling
In a small pot, heat up the milk and vanilla extract on medium heat until small bubbles start coming to the surface and it just starts to simmer.
Meanwhile, in a large mixing bowl, whisk together the eggs, sugar, cornstarch, and vanilla extract.
With one hand, slowly pour the hot milk into the egg mixture while whisking continuously and vigorously with the other hand to temper the eggs.
Pour the entire mixture back into the same pot and return it to the stovetop on medium heat. Cook the mixture while stirring often until it thickens into a custard, about 15 minutes. When you drag a spatula through the custard, it should be thick enough to part and come back together after several seconds.
Turn off the heat and stir in the cubed butter, one piece at a time, until it has all melted into the custard. If your custard is lumpy, push it through a fine-mesh sieve.
Transfer the custard to a medium bowl and press a piece of plastic wrap directly on top of the surface of the custard to prevent a skin from forming. Let it cool completely in the fridge before filling your baked tart shell.
Whipped Cream & Topping
In a large mixing bowl, use an electric hand mixer or whisk to beat whipping cream, powdered, sugar, and vanilla extract on medium speed until it becomes firm peaks.
Transfer the whipped cream to a piping bag fitted with a large round tip. Pipe the whipped cream on top of the custard in your tart.
Top the tart with fresh strawberries. Chill the tart in the fridge for at least 4 hours to let the custard firm up. Dust with powdered sugar right before serving.
Notes
Tart shell dough: This recipe makes enough tart dough for two 9" tart shells to avoid splitting an egg in half. You can freeze the second tart dough or baked tart shell for another time.