Combine lukewarm milk, sugar, and active dry yeast in a stand mixer bowl; let sit for 5-10 minutes until foamy.
Add the mashed potatoes, melted butter, and one egg to the yeast mixture and whisk until combined.
Using a dough hook attachment, gradually add the flour and salt on low speed until a shaggy dough forms.
Increase speed to medium and knead for 7-8 minutes until the dough is smooth, elastic, and clears the sides of the bowl.
Transfer dough to a lightly oiled bowl, cover with a damp cloth, and let rise in a warm spot for 60 minutes or until doubled in size.
While the dough rises, brown the ground beef in a skillet over medium heat, breaking it into small crumbles; drain excess fat.
Add the minced onion and garlic to the beef; sauté for 5 minutes until onions are translucent.
Stir in Worcestershire sauce, paprika, pepper, and additional salt to taste; remove from heat and allow to cool completely.
Punch down the risen dough and divide into 12 equal portions (approximately 75g each).
Flatten each portion into a 4-inch disc, place 30g of the beef filling in the center, and gather the edges to seal tightly.
Place the rolls seam-side down on a parchment-lined baking sheet, cover, and let rise for an additional 30 minutes.
Preheat the oven to 190°C (375°F).
Brush the tops of the rolls with the egg wash and sprinkle with sesame seeds.
Bake for 20-25 minutes until the rolls are deep golden brown and reach an internal temperature of 88°C (190°F).