In a large bowl, combine flour, sugar, salt, baking powder and baking soda with a fork.
In another bowl, beat the egg thoroughly, then add buttermilk, whole milk and melted butter; beat to combine.
Pour wet ingredients into dry ingredients and combine with a wooden spoon. DO NOT OVERMIX; it's fine if it's lumpy.
Heat a griddle or large skillet over medium heat. Melt 1 tsp butter. (You may need to turn down the heat after the first few pancakes to prevent overbrowning; you may need to add more butter from time to time if the pan gets dry.).
Using a small ladle or a pitcher with a pour spout, pour about 3 tbsp of batter for each pancake, leaving space for them to spread out. If using blueberries, scatter a few onto each cake right after pouring. When the top of the pancake no longer looks wet, the holes are just barely set, and the underside is golden (about 3 minutes), flip the pancake and cook until the underside is golden, about 2 minutes more.
If not serving immediately, keep warm in a 200°F oven until all the pancakes are cooked and ready to serve, no more than 15 minutes.