If using cornmeal instead of masa harina, grind it in a clean coffee mill or food processor until it has a texture similar to farina.

Combine flour, masa harina, cumin (optional), salt, milk, eggs, sugar (optional), and melted butter in a blender.

Blend until smooth, adding a little more milk if the batter is too thick.

Let the batter rest at room temperature for 30 minutes.

Lightly brush a 10-inch nonstick skillet with butter and heat over moderately high heat until hot but not smoking.

Stir the batter.

Remove the skillet from the heat and pour in 1/3 cup of batter, immediately tilting and rotating the skillet to coat the bottom.

If the batter sets too quickly, reduce the heat slightly for the next crepe.

Return the skillet to the heat and cook until the crepe is just set and pale golden around the edges (10-15 seconds).

Loosen the edge of the crepe with a heatproof silicone spatula.

Carefully flip the crepe over with your fingertips.

Cook until the underside is set (about 20 seconds more).

Transfer the crepe to a plate.

Repeat the process to make 11 more crepes, brushing the skillet lightly with butter for each crepe.

Stack the crepes on the plate as they are cooked.