Combine the warm water, 2 ¼ tsp levadura seca activa 1 paquete estándar, and 1 tsp 1 tsp azúcar granulada para activar la levadura in a small Mixing Bowl. Stir gently and let sit for 5–10 minutes until foamy and fragrant. If the 2 ¼ tsp levadura seca activa 1 paquete estándar doesn't foam, it may be dead — start over with fresh 2 ¼ tsp levadura seca activa 1 paquete estándar.

In the bowl of a Stand Mixer fitted with the dough hook, combine the 3 ½ cups harina de propósito general más para espolvorear, 1 tsp azúcar granulada para activar la levadura, and 1 tsp sal kosher. Whisk briefly to combine. Add the 3 huevos temperatura ambiente, ⅓ cup leche entera temperatura ambiente, and the activated 2 ¼ tsp levadura seca activa 1 paquete estándar mixture. Mix on low speed for 2 minutes until a shaggy dough forms.

Increase the speed to medium and knead for 5 minutes until the dough is smoother. With the mixer running, add the ½ cup mantequilla sin sal cortada en cubos, temperatura ambiente cubes a few at a time, waiting until each addition is incorporated before adding more. This process takes about 5–7 minutes. The dough will look shaggy at first — keep going.

Once all the ½ cup mantequilla sin sal cortada en cubos, temperatura ambiente is incorporated, increase speed to medium-high and knead for another 8–10 minutes until the dough is smooth, glossy, and pulls away cleanly from the sides of the Mixing Bowl. It will be soft and slightly tacky but should not stick to your fingers. If it's very sticky, add 3 ½ cups harina de propósito general más para espolvorear 1 tbsp at a time.

Shape the dough into a ball and place it in a lightly oiled large Mixing Bowl. Cover tightly with Plastic Wrap and let rise at room temperature until doubled in size, about 1.5 to 2 hours. Alternatively, refrigerate overnight (8–12 hours) for a slower, more flavorful rise — this also makes the dough easier to roll.

While the dough rises, make the filling. In a small Saucepan over low heat (or in the Microwave in 30-second bursts), melt the chopped 4 oz chocolate oscuro finamente picado (60–70% cacao) and ½ cup mantequilla sin sal cortada en cubos, temperatura ambiente together, stirring until completely smooth. Remove from heat.

Whisk in the sifted 1 tsp azúcar granulada para activar la levadura, 3 tbsp polvo de cacao sin endulzar tamizado, and 1 tsp sal kosher until the filling is smooth, thick, and spreadable — similar in consistency to Nutella. Set aside to cool to room temperature. It will thicken as it cools.

Combine the 1 tsp azúcar granulada para activar la levadura and ¼ cup agua tibia aproximadamente 110°F in a small Saucepan over medium heat. Stir until the 1 tsp azúcar granulada para activar la levadura dissolves completely, then bring to a brief simmer. Remove from heat and set aside to cool. This can be made ahead and stored in the fridge.

Grease a 9x5-inch Loaf Pan and line it with Parchment Paper, leaving an overhang on the long sides for easy lifting. Punch down the risen dough and turn it out onto a lightly floured surface.

Roll the dough into a large rectangle, approximately 12 inches wide by 16 inches long. Aim for an even thickness of about 1/4 inch. If the dough springs back, let it rest for 5 minutes and try again.

Spread the chocolate filling evenly over the dough, leaving a 1/2-inch border along one of the long edges. Scatter the unwrapped 4 oz monedas de chocolate gelt sin envolver; leche evenly over the filling, pressing them in lightly.

Starting from the long edge opposite the border, tightly roll the dough into a log. Pinch the seam firmly to seal. Place the log seam-side down and use a Knife to cut it in half lengthwise, exposing the swirled layers.

With the cut sides facing up, twist the two halves around each other 3–4 times to form a braid, keeping the layers visible on top. Gently tuck the ends under and transfer the twist into the prepared Loaf Pan.

Cover the pan loosely with Plastic Wrap and let the shaped babka rise at room temperature for 45–60 minutes, until it's puffed and nearly reaches the top of the pan. Meanwhile, preheat the Oven to 350°F.

Brush the top of the babka gently with the beaten 3 huevos temperatura ambiente wash. Bake at 350°F for 40–45 minutes, until deep golden brown on top. If the top browns too quickly, tent loosely with Aluminum Foil after 25 minutes. The internal temperature should reach about 190°F.

As soon as the babka comes out of the Oven, brush it generously with the simple syrup — use all of it. This gives the babka its signature glossy, moist, bakery-quality finish. Let it cool in the pan for 20 minutes, then lift it out using the Parchment Paper overhang and cool on a Wire Rack for at least 30 minutes before slicing.