6 Tbsp. (100 g) sourdough starter
2 cups (250 g) all-purpose flour, divided
2 large eggs
¾ cup plus 1 Tbsp. (or more) milk
3 Tbsp. granulated sugar
½ tsp. baking soda
½ tsp. kosher salt
3 Tbsp. unsalted butter, melted, plus more room temperature
Grapeseed or vegetable oil (for pan)
Jam, crème fraîche, demerara sugar, and/or lox, (for serving; optional)
Mix starter, 1 cup plus 6 Tbsp. (172 g total) flour, and ¾ cup plus 1 Tbsp. (193 g) room-temperature water (about 80°F) in a medium bowl until no lumps remain. Cover and let sit at room temperature overnight.
Fold in remaining ½ cup plus 2 Tbsp. (78 g total) flour. Then add eggs, milk, granulated sugar, baking soda, salt, and 3 Tbsp. melted butter and whisk, adding more milk if needed, until mixture is thick but pourable (you want it to be about the consistency of heavy cream). It will take awhile to work out lumps—a few small lumps are fine, but try to get batter as smooth as possible.
Pour in enough oil to lightly coat a crepe pan or 10" nonstick skillet and heat over medium-high (this is just to help get things going; you shouldn’t need to add more oil to the pan). Pour ¼ cup batter into center of pan and tilt and rotate pan as needed to coat bottom evenly with batter. Cook crepe, undisturbed, until surface looks dry and bottom is golden brown, about 45 seconds. Using tongs or an offset spatula, gently flip crepe and cook just until other side is golden, about 20 seconds. Transfer to a plate and slather a bit of room-temperature butter on top. Repeat process with remaining batter, stacking and buttering crepes as you go.
Serve crepes with jam, crème fraîche, demerara sugar, lox, and/or more butter as desired.