In a large Mixing Bowl, whisk together the 1 ¼ cups harina de trigo plus más para espolvorear, ¾ cup azúcar granulada, and 1 pinch sal marina fina. Add the cold 4 tbsp mantequilla sin sal cortada en trozos pequeños cubes and use your fingertips (or a pastry cutter) to rub the 4 tbsp mantequilla sin sal cortada en trozos pequeños into the 1 ¼ cups harina de trigo plus más para espolvorear until the mixture resembles coarse, pea-sized crumbs — some larger bits of 4 tbsp mantequilla sin sal cortada en trozos pequeños are fine and desirable.

Drizzle in the 3 tbsp agua helada más si es necesario one tablespoon at a time, stirring with a fork after each addition, just until the dough begins to clump together. It should hold when pressed but not feel wet or sticky. Tip it onto a lightly floured surface and bring it together into a flat disk. Wrap in Plastic Wrap and refrigerate for at least 20 minutes (or up to 2 days).

Peel, core, and quarter the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos. If using large 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos, cut each quarter in half lengthwise (into eighths). Pat them dry with paper towels — removing excess moisture helps the caramel concentrate properly.

Place a 10-inch oven-safe Skillet (cast iron or stainless steel) over medium heat. Spread the ¾ cup azúcar granulada in an even layer across the bottom. Cook without stirring, swirling the pan gently once or twice, until the ¾ cup azúcar granulada melts and turns a deep amber color, about 8–10 minutes. Watch carefully — it can go from perfect to burnt quickly.

Remove the pan from heat and immediately add the 4 tbsp mantequilla sin sal cortada en trozos pequeños pieces and 1 pinch sal marina fina. Stir carefully — the mixture will bubble vigorously. Return to low heat and stir until the 4 tbsp mantequilla sin sal cortada en trozos pequeños is fully melted and the caramel is smooth. Stir in the 1 tsp extracto de vainilla pura if using.

Arrange the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos pieces tightly in the caramel, standing them on their edges in concentric circles, packing them in snugly — they will shrink as they cook. Start from the outside edge and work inward. It will look like too many 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos at first; that's exactly right.

Cook the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos over medium heat for 20–25 minutes, occasionally pressing them down gently with a Spatula, until the caramel is thick and bubbling around the edges and the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos are just tender when pierced with a knife. Remove from heat and let cool for 10 minutes.

While the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos cook, preheat your Oven to 400°F. On a lightly floured surface, roll the chilled dough into a circle roughly 11–12 inches in diameter (about 1/8 inch thick) — just slightly larger than your Skillet.

Drape the pastry circle over the Rolling Pin and carefully lay it over the 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos in the Skillet. Tuck the edges of the dough down around the inside of the pan, like tucking in a blanket. Use a fork or knife to poke 4–5 small steam vents in the top.

Transfer the Skillet to the Oven and bake for 25–30 minutes, until the pastry is deep golden brown and the caramel is visibly bubbling at the edges.

Remove from the Oven and let the tart rest in the pan for exactly 5–10 minutes — no longer, or the caramel will set and the tart will stick. Place a large, flat serving plate (with a rim to catch any caramel drips) upside-down over the Skillet. In one confident motion, flip the Skillet and plate together. Lift the Skillet off slowly. If any 2 ½ lb manzanas firmes como Granny Smith, Braeburn o Honeycrisp — peladas, descorazonadas y cortadas en cuartos pieces have shifted, use a spoon to nudge them back into place.

Serve warm, cut into wedges, with a scoop of 1 cup helado de vainilla o crème fraîche or a generous spoonful of crème fraîche. The tart is best enjoyed within a few hours of baking.