Mix the flour, buttermilk (or kefir), and sourdough starter in a large bowl until combined. Cover and leave at room temperature overnight for 8-10 hours.
In the morning, the batter should look slightly bubbly. Beat the eggs separately, then add them to the bowl along with the honey, melted butter, salt, vanilla, and baking soda. Gently mix everything together with a spatula until just combined. Do not overmix.
Heat a pan over medium heat and add a little butter for the first pancake. Use about ¼ cup of batter per pancake. A large cookie scoop or ladle works well. Pour in the batter and cook until bubbles form on the surface and the edges look set, about 1 to 1 ½ minutes. Flip and cook the other side for another minute until golden and cooked through.
Repeat with the remaining batter and serve warm.
