In a large bowl or the bowl of a stand mixer, combine the flour, instant yeast, sugar, and salt. Add the warm water and the oil, lard, or butter. Mix until a shaggy dough forms. Knead for about 10 to 12 minutes by hand or 8 to 10 minutes in a stand mixer until the dough is smooth, elastic, and only lightly tacky. The dough should feel soft and supple, not dry and stiff. If it feels very sticky, resist adding much extra flour. A slightly soft dough gives the bolillos their lighter interior. That extra retained moisture is directly what allows the crumb to bake up genuinely soft and airy — a dough worked drier for easier handling would trade away exactly that lightness.

Shape the dough into a ball and place it in a lightly oiled bowl. Cover and let rise in a warm spot for about 1 hour, or until doubled in size. The dough should look puffy and airy and should slowly spring back when pressed lightly with a finger. That slow spring-back is a genuine, reliable readiness cue — dough that springs back instantly hasn't yet built enough structure, while dough that doesn't spring back at all has likely overproofed.

Turn the dough out onto a lightly floured surface and gently press out the biggest air pockets. Divide into 8 equal pieces, about 105g each. Shape each piece into a loose round and let them rest, covered, for 10 minutes. This short bench rest relaxes the gluten and makes the final shaping easier. If skipped, the dough may resist stretching into the classic bolillo shape. That relaxation matters directly at the next stage — dough that's just been divided and shaped is genuinely tighter and more resistant to the specific folding and rolling the bolillo shape requires.

Working with one piece at a time, flatten it gently into a small oval. Fold the top third down and seal lightly, then fold the bottom third up and seal again. Roll the dough gently under your palms into a short oval roll with slightly tapered ends. Pinch the seam closed and place seam-side down on a parchment-lined baking sheet. Repeat with the remaining dough pieces, spacing them apart. Bolillos should be compact, oval, and slightly pointed at the ends, not long like baguettes and not round like burger buns. Placing each shaped roll seam-side down protects that pinched seal from opening during proofing and baking, letting the roll's own weight keep it pressed closed until the crust sets.

Cover the shaped rolls loosely with a towel or lightly greased plastic wrap and let proof for 35 to 45 minutes, until puffy but not overexpanded. Meanwhile, preheat the oven to 220°C. Place a small metal pan on the lower oven rack so you can create steam later. Do not overproof. If the rolls rise too much before baking, they can spread out and lose the classic bolillo shape. That specific stopping point — puffy but not overexpanded — protects the rolls' compact, tapered silhouette, since dough given too much time to rise before baking naturally spreads and softens its own defined shape.

Right before baking, use a very sharp knife, lame, or razor to make one long lengthwise slash across the top of each roll. The cut should be fairly confident and centered. The slash controls expansion in the oven and gives bolillos their classic look. A timid shallow cut may not open properly. A confident, deep-enough slash gives the dough's own oven-spring a controlled, predictable place to expand — a shallow or hesitant cut instead forces that expansion to find its own way out, often producing a less clean, less classically shaped result.

Place the tray in the oven. Carefully pour a little hot water into the hot pan on the lower rack to create steam, then close the oven door quickly. Bake for 18 to 22 minutes, until the bolillos are lightly golden, feel light for their size, and sound slightly hollow when tapped underneath. The crust should be light golden and thin, not dark brown and thick. Bolillos are usually lighter than rustic European rolls. That early steam keeps the dough's own surface soft and pliable for longer during the first minutes of baking, which is directly what allows the bread to expand fully in the oven before the crust sets and hardens into its final, thin form.

Transfer the bolillos to a wire rack and cool for at least 20 minutes before slicing. If sliced too early, the crumb can compress and seem gummy. Letting them cool allows the interior to set properly while the crust stays delicate. That cooling period lets residual steam trapped inside the crumb finish escaping and the starch structure fully set — bread sliced while still hot hasn't finished this internal process, and the knife's pressure compresses a structure that hasn't yet firmed up enough to hold its own shape.