Make the crust: Add the cornflakes to a zip-top bag, seal, and crush into crumbs with a rolling pin.
Transfer the crumbs to a large bowl and add the melted butter and sugar. Stir to combine.
Add the crumb mixture to a 9½-inch (24 cm) pie dish and press evenly into a ¼-inch-thick (6 mm) layer against the bottom and up the sides of the pan.
Bake until the crust is golden brown, about 15 minutes. Let cool completely.
Make the filling: In a small saucepan over medium heat, bring the milk to a simmer.
Add the Thai tea leaves and stir into the hot milk. Turn off the heat and let the tea steep for 30 minutes.
In a medium bowl, whisk together the egg yolks, salt, cornstarch, and sugar.
Line a fine-mesh sieve with cheesecloth and set over the bowl with the egg mixture. Slowly pour in the Thai tea mixture, pressing the leaves to extract as much liquid as possible. Whisk to combine.
Return the Thai tea mixture to the saucepan over medium heat and cook, stirring constantly, until thickened, about 8 minutes. Add the butter and whisk until melted and the filling is smooth. Remove from the heat.
Brush the chocolate hazelnut spread over the bottom of the cooled crust. Sprinkle the toasted coconut flakes over the chocolate hazelnut spread.
Pour the Thai tea filling into the crust and spread evenly. While still hot, cover the pudding with plastic wrap to prevent a skin from forming . Refrigerate the pie until set, at least 4 hours.
Before serving, remove the plastic wrap and spread whipped cream over the top of the pie. Serve chilled.