Preheat the oven to 300℉.
In a small saucepan, heat the cream and milk over medium heat until steaming, about 5 minutes.
Fill a large bowl with ice and water and set it nearby. In a separate large bowl, whisk together the egg yolks and superfine sugar until combined, about 3 minutes. While whisking continuously, slowly pour a ladleful of the hot cream into the egg mixture and whisk vigorously to combine. Pour the egg mixture into the saucepan with the cream mixture. Bring to a simmer and cook until the temperature reaches 184℉. Place the pan over the ice bath to cool completely.
Transfer the mixture to a blender and add the lemon juice, avocado, and vanilla. (Fun science note: Adding the avocado when the mixture is cool prevents browning. And adding the lemon prevents. the mixture from turning into clotted cream. In other words, don't skip this step unless you want to make avocado cheese.) Blend until smooth, then strain the mixture through a fine-mesh sieve into a large liquid measuring cup or other vessel with a spout.
Pour the mixture into four small ramekins and set the ramekins in a roasting pan. Run water from the tap until the water is very hot. Add enough hot water to the roasting pan to come halfway up the sides of the ramekins, being careful not to get any water in the ramekins. The ramekins should not be floating. Carefully transfer the roasting pan to the oven and bake for 30 to 45 minutes, until the custard has set into a soft butt wobbly-in-the-center texture. Remove from the oven and remove the ramekins from the roasting pan. Refrigerate until completely chilled before serving, at least 4 hours or up to overnight.
Just before serving, sprinkle brown sugar over the surface of each custard and use a kitchen torch to melt and crystallize the sugar into a hard shell. (Alternatively, you can slide them directly under the broiler for 1 to 2 minutes to brûlée the sugar—just keep an eye on them.