Combine whole-wheat flour, all-purpose flour, sugar (if using), salt, eggs, milk and butter (or oil) in a food processor or blender; process until smooth, scraping the sides once or twice. Transfer to a bowl, cover and refrigerate for at least 30 minutes or overnight.
Slowly whisk seltzer water (or club soda) into the crepe batter. Lightly oil a small nonstick skillet with a crumpled paper towel; heat the skillet over medium-high heat. Ladle about 2 tablespoons batter into the skillet with a small ladle or measuring cup; immediately tilt and rotate the pan to spread the batter evenly over the bottom.
Cook the crepe until the underside is lightly browned, about 30 seconds. Using a small metal spatula or a fork, lift the edge of the crepe, quickly grasp it with your fingers and flip. Cook until the second side is lightly browned, about 20 seconds longer. Slide the crepe onto a plate.
If serving immediately, fill and roll or fold the crepe. Continue to cook crepes with the remaining batter, oiling the pan as needed and stacking crepes as you go.Variations:Buckwheat Crepes: Substitute buckwheat flour for the whole-wheat flour. Fill with sauteed apples or smoked salmon and cream cheese.Cornmeal Crepes: Substitute cornmeal for the whole-wheat flour. Try with a spicy chili or with blueberries and cream cheese.
Buckwheat flour is available at natural-foods stores or by mail-order from Bob's Red Mill, (800) 349-2173, www.bobsredmill.com.