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Prepare a baking sheet by greasing it with nonstick cooking spray or butter. For this step, you could line the sheet with parchment paper, instead, if you would prefer an easier clean-up. The slices should be about 1/2 inch (1.25 cm) thick. Add butter to both sides and arrange them on your prepared baking sheet.  You will end up using roughly half of the butter, or 1/4 cup (60 ml), for this step. Bolillo rolls are a type of Mexican bread available through many market bakeries. If you cannot find bolillo, though, French bread makes an excellent substitution. Both types of bread have crusty outsides and soft insides.  For best results, use bread that has been left out or bread that is at least 2 days old. This bread is already somewhat stale, but for a recipe like this, a little extra crispiness is actually preferable. When ready, both sides should be dry and lightly browned.  Remove the rolls from the baking sheet and let cool. Set aside. Turn the rolls after 3 minutes using tongs. Work carefully to avoid getting burned. Toasting the bread in the oven is the contemporary way to prepare the bread, but a more traditional way to prepare it would be by frying it on the stovetop in a cast iron skillet.  Melt 1 to 3 Tbsp (15 to 45 ml) of butter in the skillet over medium heat. Make sure that the butter coats the entire bottom of the skillet. Butter both sides of the each slice of bread, as with the oven-toasted method. Add the pieces of bread to the skillet a few at a time. Do not overcrowd the pan. After 3 minutes or so, use a spatula to flip the bread over to the other side. The side that initially faced the pan should be crispy and lightly browned. Give the other side an additional 2 to 3 minutes to cook. When done, that side, too, should be crispy and lightly browned. Note that you may need to add more butter as you go along to keep the bottom of the skillet well-coated.
Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Cut the rolls into thin slices. Bake the rolls for 3 minutes per side. Alternatively, fry the bread.