2 teaspoons (10 milliliters) lime zest, divided, plus more for garnish
1/4 teaspoon (1.25 milliliters) salt
2 sheets frozen puff pastry, thawed
1 cup (240 milliliters) blackberries
1 tablespoon (15 milliliters) milk
Preheat oven to 350 F (175 C).
Using a hand mixer, beat together mascarpone and sugar until light and fluffy. Add egg yolks, one teaspoon (five milliliters) lime zest, and salt, and mix until combined.
Unroll both sheets of puff pastry on a work surface, then roll out into two 25.5 x 25.5 centimeter squares. Cut each square into four squares. Scoop a heaping tablespoon of the mascarpone mixture into the middle of one of the squares, then top with blackberries. Grab two opposing corners of the pastry and fold up to meet each other in the middle, over the filling. Pinch together to secure. Place on baking sheet. Repeat until all the danishes are filled and folded.
In a small bowl, whisk together egg and milk to make an egg wash, then brush over the pastry. Place in the oven to bake for 20-24 minutes, until golden and flaky. Remove from oven, garnish with lime zest, and serve.