In a mixing bowl, sift together the plain flour, powdered icing sugar, and salt. Mix in the ground Earl Grey tea leaves and cubed unsalted butter using fingertips until resembling coarse crumbs. Add the large egg and ice-cold water, mixing until a dough forms. Chill for 1 hour.

Roll out the chilled dough on a floured surface until 1/8 inch thick. Place the dough into a tart tin, pressing gently into the edges. Trim excess dough and refrigerate the tart shell for 30 minutes.

Preheat oven to 375°F (190°C). Dock the bottom of the tart shell with a fork, line with parchment paper, and fill with baking weights. Bake for 15 minutes, then remove the weights, brush with egg wash, and return to oven for 10 minutes.

Gently heat the double cream in a saucepan until steaming (not boiling). Add Earl Grey tea bags and steep for 20 minutes; remove tea bags and set cream aside to cool slightly.

In a bowl, whisk together egg yolks and caster sugar until pale and creamy. Gradually pour warm steeped cream into yolk mixture while whisking continuously. Strain custard through a fine mesh sieve into a jug.

Preheat oven to 300°F (150°C). Pour Earl Grey custard into the baked tart shell and bake for approximately 40 minutes until just set. Let cool completely at room temperature.

Once cooled, sprinkle caster sugar over the custard and caramelize using a kitchen blow torch until golden brown and bubbly.