Use a ski-type hat as a pattern for the main shape of the mask. Place it on the polar fleece and trace around it. Make it longer than the cap by extending it down a few inches longer than the ski cap. (This is because it will be worn over the eyes.)
Flip the cap right side out and try it on.
Batman's mask has horn-like projections coming up from either side. Draw them on your pattern freehand.
Cut out the drawn pattern twice. You want 2 identical pieces.
Place one piece over the other and sew around all edges except for the bottom.
Mark where the eyes and nose are when the mask is worn.
Cut out holes for the eyes and cut out the bottom of the mask, leaving room for the nose.
Fill the "horns" with fabric scraps so that they will stay up when the mask is worn.