In a bowl with an electric whisk - or a stand mixer - cream together the butter, sugar, lemon zest until fluffy, about 1-2-minutes.
Add the egg yolks into the batter and beat to combine fully. Add the flour & beat until a dough forms. Use your hands to form it into a disc shape, squeezing any loose crumbs together, and wrap it tightly in a layer of cling film. Leave to chill in the fridge for at least 20-minutes.
Lightly grease a 9-inch tart tin with butter and set aside. Preheat the oven to 180°C.
Once chilled, roll the dough out on a lightly floured surface, to about 4mm thick, turning the pastry often. Line your prepared tin with the pastry, trimming off any excess, and pop in the freezer for 15-minutes or until very cold.
Cover the frozen pastry with a layer of baking parchment, fill with baking beans/rice/etc, and blind bake in the preheated oven for 15/20-minutes, or until you see the edges of the tart shell go lightly golden in colour. Remove the parchment paper + baking beans and return the tart shell to the oven for a further 15/20-minutes, or until the base of the tart is golden brown. Leave to cool for 5-minutes before brushing with remaining egg measurement and returning to the oven for a further 4-minutes. Leave to cool completely.