Mix flour and egg with 1/4 cup of the milk, stirring with whisk until batter is smooth.
Mix in remaining milk, add rum or vanilla, sugar and salt, and stir until well combined. (Should measure about 1 1/3 cups.) Divide butter into 4 equal pieces. Melt 1 piece in large 10-12 inch skillet. When butter is hot, add about 1/3 cup batter; swirl pan to coat bottom. Cook over medium to high heat for about 2 minutes on first side (until nicely browned) then turn and cook for 1 minute until second side is done. Repeat with remaining butter and batter to make 3 more crepes. Fill crepes while warm and roll. Dust with powdered sugar. Serve with sour cream, whipped cream or a combination of both, if desired.