1Make Lemon Pastry Crème:
In a medium saucepan over medium heat, combine the milk, lemon zest, sugar, butter and salt. Cook just until mixture begins to boil and remove from heat. In a small bowl whisk egg yolks, vanilla and lemon juice. Temper the egg mixture with a spoonful of the milk mixture, whisking. Slowly add egg mixture to milk mixture in pan, whisking constantly until smooth and combined. Return the mixture to low heat and whisk for 2-3 minutes, until thick and glossy. Remove from heat and place in a chilled bowl. Cover with plastic wrap, making sure it touches the top of the pastry crème, and chill while making profiteroles.
Preheat oven to 400 degrees. Line 2 baking sheets with parchment paper.
In a medium saucepan, combine the water, butter, sea salt and butter and cook on high until boiling. Remove from heat and add the flour all at once, stirring vigorously with a wooden spoon until a ball forms. Let mixture cool for 2-3 minutes. Add eggs, 1 at a time, beating well after each one, until mixture is smooth and glossy. Drop by large spoonfuls onto prepared baking sheets, making 24 equal mounds, approximately 2 inches in diameter. Bake in preheated oven for 14-15 minutes, or until golden and hollow sounding when touched. Place on cooling rack, poke a hole in the top portion of each to release steam and cool completely. Leave oven on to brown tops when filled.
In the bowl of an electric mixer with the whisk attachment, beat egg whites and cream of tartar on high until soft peaks form. Slowly add sugar and continue to beat until stiff peaks form.
Remove pastry crème from refrigerator and remove plastic wrap. Carefully split the top portion of each profiterole, leaving one side attached. Either pipe or spoon in pastry crème, filling to the top of each. Place tops back on and leave on baking sheets. Top each with meringue. Turn the oven to broil and place one baking sheet in at a time, on upper middle rack. Leave door cracked and brown tops of meringue lightly, watching constantly so they won’t burn. Remove and repeat with other baking sheet. Serve immediately.