For the lemon curd, whisk egg yolks, eggs, sugar, zest and lemon juice in a bowl over a pot of gently simmering water until thick (thick enough so you should be able to spoon it, not pour it). Remove from heat and scrape into a food processor. Pulse in butter, sour cream and table cream until smooth. Scrape curd into a bowl and cover surface with plastic wrap. Chill for at least 3 hours.
For crème, Heat milk and cream in a saucepan. In a bowl, gently whisk eggs, egg yolks, sugar and salt. Ladle about ½ cup of milk mixture into eggs, whisking in. Continue adding milk and whisking until it has all been blended in. (Do not whisk too vigorously). Stir in liqueur, if adding, and strain. Chill completely.
Preheat oven to 350 degrees F. and arrange eight 6-ounce tempered glass dishes or ceramic ramekins on a tea towel in a baking dish with a 2-inch lip.
Spread a large spoonful of lemon curd at the bottom of each baking dish or ramekin. Carefully pour the chilled custard over the lemon curd, dabbing off any bubbles with the edge of a paper towel. Pour boiling water around baking dishes or ramekins. Cover pan loosely with foil and bake for 35 to 40 minutes, until custards are set but center still jiggle when tapped.
Remove from water bath, let cool for 20 minutes, then chill completely, at least 4 hours.
For brulée, place sugar and water in a saucepan over high heat and boil, without stirring, until a light amber colour. While cooking, occasionally brush down the sides of the pot with a clean brush dipped in water. Immerse pot in a bowl of cold water to halt cooking, then spoon a thin layer of caramel over each cooled custard, swirling cup to coat evenly.
The Crème Brulées can be returned to the fridge and served up to 3 hours later.