Buckwheat Crêpes With Creamy Leeks and Baked Eggs

Whisk eggs, sugar, and salt in a large bowl to combine. Whisk in milk, followed by all-purpose flour, then buckwheat flour. Blend batter with 4 Tbsp. butter just to incorporate. Strain through a fine-mesh sieve into a medium bowl. Cover and chill batter at least 12 hours to allow flour to hydrate. Let batter sit at room temperature 1 hour before cooking. Stir briskly to reincorporate flour that will have settled to the bottom. Batter should be the consistency of heavy cream (adjust with more flour if too thin or milk if too thick). Heat a 10" nonstick skillet over medium. Brush skillet lightly with remaining 1 Tbsp. butter. Lift pan away from heat and pour 1/4 cup batter in the middle of skillet and quickly swirl pan to distribute batter evenly. Cook until crêpe begins to set and edges brown, about 2 minutes. Carefully loosen with a heatproof rubber spatula, flip, and cook until other side is lightly browned, about 1 minute. Stir batter to reincorporate; make another crêpe with 1/4 cup batter. (Recipe yields enough batter for 8 crêpes; reserve extra for coming days.)