Combine masa harina and boiling water in the bowl of a stand mixer. Mix until crumbly and set aside to soften for a few minutes. Combine bread flour, gluten, sugar, salt, and yeast in a small bowl.
Pour milk into the bowl with the masa; add flour mixture and lard. Mix on low speed with the dough hook until dough is well combined and clears the sides of the bowl, about 2 minutes. Scrape dough down and mix for 5 minutes more to develop the gluten; dough will be very stiff.
Transfer dough to a lightly oiled bowl and cover with plastic wrap. Let rise at room temperature, about 1 1/2 hours. Place butter in the same mixing bowl and use the paddle attachment on low speed to break it up. Mix just until smooth.
Scrape butter onto a piece of parchment or waxed paper. Form into a 7-inch square. Cover with another piece of parchment and smooth the top with a rolling pin. Re-form the edges of your square if necessary; a little fussing now will save you some trouble later on as you roll out the dough. Place butter square in the refrigerator to chill.
Transfer dough onto a floured work surface. Pat it into a square, transfer to a parchment-lined baking sheet, cover with plastic wrap, and refrigerate for 1 to 24 hours.
Remove butter square from the refrigerator about 20 minutes before you plan to roll the dough; it needs to be cool and pliable.
Transfer dough onto a floured work surface. Roll it into a square a little larger than the butter square. Leaving a 7-inch square platform in the center for the butter, roll each of the 4 corners of dough out into elongated flaps that can wrap around the butter like a present.
Place butter square in the center of the dough, fold up the flaps, and pinch together to seal. Roll dough out into a rectangle. Fold in thirds, like a business letter. Wrap in plastic wrap and refrigerate for 30 minutes.
Return dough to the work surface; dust with flour if needed. Roll dough out into a longer rectangle; you're going to do a 4-fold, also called the "book fold." Fold the left edge of the dough into the center of the rectangle; fold the right side in to meet it. Then fold it in half again, like a wallet. Return dough to the parchment-lined baking pan, cover with plastic wrap, and refrigerate for 30 minutes.
Place dough on the work surface, roll out into a rectangle again, and make the final turn: another 3-fold, like a business letter. Cover and return to the refrigerator for 30 minutes; the dough can also be frozen at this point.
Slice dough in half and return 1 piece to the refrigerator to stay cool. Roll dough out to 1/4-inch thickness. Use a cutter to create 5-inch circles. Transfer circles to a parchment-lined baking sheet and let rise for 1 hour. Shape dough scraps into "tacro cups" and place in the cups of a muffin tin; let rise. Repeat both steps with remaining dough.
Heat oil in a deep-fryer or large saucepan to 350 degrees F (175 degrees C). Gently fold a dough circle in half to form a taco shell shape; holding it with tongs, lower it into the oil. Hold it closed for about 10 seconds. Fry 2 shells at a time until golden, turning once, about 3 minutes total. Repeat with remaining dough circles, maintaining the oil heat between 350 and 375 degrees F (190 degrees C). Drain tacro shells on paper towels.
Preheat the oven to 375 degrees F (190 degrees C).
Bake tacro cups in the muffin tin until golden, 15 to 18 minutes. Fill cups and shells with your favorite taco fillings.