Filling: In a small saucepan, add the potato and enough cold water to cover. Add a generous pinch of salt and bring to a boil over medium heat, then reduce the heat and simmer until tender, 12 to 15 minutes.
In a second small saucepan, bring 4 cups water to a simmer and add a generous pinch of salt, bay leaves, 1 teaspoon coriander seeds, peppercorns, chili flakes, and chicken breast. Simmer until cooked through, 8 to 10 minutes.
Mash the potato and finely shred the chicken and put into a large bowl. Combine with mango, lime juice, chipotle sauce, remaining coriander seeds, cumin, cilantro, and salt and pepper, to taste. Mixing with a spoon or your hands, until well incorporated. Taste for seasoning and adjust, if necessary. Set aside.
Samosas: Roll the puff pastry out slightly, to flatten the seams. Using a paring knife to trace a 7-inch ring mold or bowl, cut the dough into 8 (7-inch wide) rounds. Cut each into 2 semicircles.
Put a tablespoon or so of the filling in the center of a semicircle. Have a small bowl of water handy. Dip your finger in the water and run it along the edges of the semicircle. Arrange the samosas so the flat side is facing away from you. Grab the left corner and fold it over the dough in a triangular motion, so that this corner lands on the bottom right side of the filling. Do the same with the other corner.
Squeeze bottom shut, and fold over, sealing with water. If you like, seal using a fork. Repeat with remaining dough and filling. Arrange the 8 samosas on a lightly greased baking sheet.
In a small bowl, whisk the egg and 1 teaspoon water with a fork until thoroughly combined. Brush the tops of the samosas with the egg wash.
Bake for 15 minutes at 425 degrees F, then turn heat down to 375 degrees F and bake for 10 more minutes. You can flip them over just before you turn the heat down, if you like.
Chutney: In a food processor, whiz together the mint, cilantro, ginger, lime juice and water until it all comes together as a sauce. There'll probably still be little bits of leaf in the sauce, but I like how those taste, so no worries. Pour the chutney into a bowl.
In a small pan, over medium heat, warm the oil until it shimmers. Add mustard seeds; they should sizzle. Immediately cover with a lid until they stop spluttering. Make sure they don't burn! If they do, start over. No big deal. I do it all the time! Immediately add the seeds and oil to the chutney. It will sizzle so stand back. Once you've poured in all of the oil, you can even spoon a little chutney into the pan (cue sizzling again!) so that you pick up all that lovely oil. Pour into the bowl with the chutney.
Serve samosas hot, with the chutney... and bask in gratitude for the great ideas we all come up with, for our sensitive taste buds and for the blessing of good friends!
Homemade Samosa Pastry:
In large bowl, combine the flour, buttermilk, oil, salt and ajwain seeds. Bring the ingredients together until a dough forms. Knead until it has softened a bit, about 5 minutes. Allow to rest, at room temperature, for 15 minutes. You can put this in the refrigerator, but make sure you let it sit at room temperature for 20 minutes or so, in order for it to soften.
Roll the dough into a short cylinder. Slice in half, then slice each half into 2 pieces, so you have 4 bits. Roll each bit into a ball. Flatten the balls into discs then, on a floured surface, roll them into a 1/8-inch thick, 7-inch wide circles. Cut in half to form 2 semicircles.