Heat the oven to 170 C / 340 F. Line two baking sheets with parchment paper.

Grind the Earl Grey tea leaves in a spice grinder or mortar and pestle until they resemble a coarse powder with no visible whole leaves.

Beat the butter and powdered sugar together in a stand mixer or with a hand mixer on medium speed for 2 to 3 minutes, until pale and creamy.

Add the ground Earl Grey, lemon zest, vanilla extract, and salt. Mix on low for 30 seconds until combined.

Add the egg yolk and mix briefly until just incorporated.

Add the flour all at once and mix on low until the dough just comes together and pulls away from the bowl sides. Do not overmix.

Turn the dough out onto a lightly floured surface and shape it into a log about 5 cm / 2 inches in diameter. Wrap tightly in plastic wrap.

Refrigerate the dough log for 20 minutes until firm enough to slice cleanly.

Unwrap the chilled log and slice into rounds about 8 mm / 1/3 inch thick using a sharp chef's knife. Place rounds 3 cm apart on the prepared baking sheets.

Bake for 12 to 14 minutes, until the edges are just barely golden and the centers look set. The tops will stay pale.

Remove from the oven and leave cookies on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.