In a large bowl, stir together 1 cup sourdough starter, buttermilk, flour, and sugar. This will be your overnight sponge. Cover it with a plastic wrap and leave at room temperature overnight, or for about 10 hours.
Then, whisk the eggs, melted butter, and salt in a small bowl. Pour in the egg mixture and 1 teaspoon baking soda into your overnight sponge and stir well to combine. The batter will start to bubble, but that's what we want. You'll get a sticky and quite thick batter.
Preheat 1/3 cup of the oil on a medium frying pan over medium-high heat. Drop a scoop of the batter into the hot frying pan and cook over medium heat until the bottom is crispy and golden brown and the top is not runny anymore, about 3 minutes, then flip on the other side and fry for 2 to 3 more minutes, or until crispy and golden. Transfer to a warm oven or platter. Repeat using the remaining oil and cooking the remaining batter.
Additional Info
To serve these pancakes with savory toppings, such as smoked salmon or caviar, add 2 tablespoons sugar instead of 3.