Preheat the oven to 120°C/Fan 100°C/Gas ½ and put a baking tray with a wire rack inside in the oven for keeping the pancakes warm. In a large bowl, whisk together the flour, baking powder, bicarbonate of soda, salt and sugar. In a separate bowl gently whisk the buttermilk with the 4 tablespoons butter and the egg yolks. Put the egg whites in a clean, grease-free bowl and whisk until they form medium peaks yet are still soft in the middle – do not over-whisk. Gently fold half the egg whites into the batter using the whisk at first and then a spatula, turning the bowl while folding. Add the remaining egg whites in the same way. The batter should be slightly lumpy for the best results, with large parts of egg white not fully incorporated.
Heat a non-stick crêpe pan over a medium heat until hot. The pan is hot enough when water sprinkled onto the surface bounces and splutters; it is too hot if it evaporates instantly. Turn the heat down to medium–low. Brush the pan with 1 tablespoon melted butter and wipe off any excess with kitchen paper. (Using butter ensures an extra-crisp finish).
Drop 2–3 tablespoons of the batter into the pan from the tip of the tablespoon for each pancake. This is the stage to scatter on the blueberries, if using. Cook for 2 minutes or until bubbles start to form and pop on the surface of the pancake and it is slightly dry and airy around the edges and lightly golden underneath. Flip over and cook for 2 minutes or until lightly golden. Keep the pancakes warm on the rack in the oven. Repeat with the remaining batter, adding more butter to the pan as necessary.
Meanwhile, fry the bacon in a dry frying pan over a high heat, or under a preheated grill, for 5 minutes. Mash up the avocados with a fork, then mix in the lime zest and juice, and season with black pepper.
To make the maple salted-butter sauce, if using, heat the maple syrup in a small saucepan over a medium heat. Whisk in the cold butter until it is incorporated and the sauce is smooth. Remove from the heat and keep warm until ready to use.
Serve a stack of pancakes, topped with the smashed avocado, crisp bacon and an indecent amount of maple syrup drizzled over, or serve with the maple salted-butter sauce.