To make the pastry, beat the butter, half the yolk, sugar and a pinch of salt together with an electric whisk, then gradually whisk in the milk. Tip in the flour and rub together with your fingertips until it just starts to come together. Squash the dough into a ball, wrap in cling film and chill for at least 1 hr. Can be made up to three days ahead.
On a lightly floured surface, roll out the dough to the thickness of a £1 coin. Use it to line a loose-bottomed 23cm tart tin, leaving the excess hanging over the sides, then prick the base all over with a fork. Return to the fridge, or better still the freezer, and leave to rest for another 30 mins.
Heat oven to 160C/140C fan/gas 3. Line the tart with baking parchment and fill with ceramic baking beans, rice or dried pulses. Place on a baking tray and bake for 10 mins, then remove from the oven. Discard the paper and bake for another 20 mins. Brush the cooked pastry with the remaining half of the egg yolk, then return to the oven for 5-10 mins until golden. Lift the pastry out onto a cooling rack, and trim the overhang. Once cooled, put the case on the baking tray.
Meanwhile, make the filling. Put the lemon verbena and cream in a pan, bring to the boil, then turn off the heat and leave to cool. Whizz the cream and leaves together using a hand blender, then pass through a fine sieve to remove any strands of verbena leaf. Whisk the cream with the rest of the filling ingredients, then pour into the pastry case. Put in the oven, making sure not to spill. Bake for 30-35 mins until just set with a slight wobble. Allow to cool. Dust with icing sugar and sprinkle over the zest. Serve the tart in slices with raspberries.