In a large bowl, whisk the flour, sugar, and salt. Add the cold butter and mix until it resembles a pebble-like or sand texture.
Combine the milk and egg yolk. Pour it into the flour mixture and mix until a dough forms. Transfer into a flat surface and knead for 5-10 minutes until smooth. Wrap in plastic wrap and put inside the refrigerator for at least an hour.
Heat a pan over medium heat. Combine the strawberry, mango, and sugar. Cook for two minutes until the sugar is dissolved. Pour the cornstarch slurry, stir, and cook until the sauce thickens. Set aside and allow the filling to cool.
Divide the dough into two and flatten using a rolling pin. The dough should not be too thin when flattened. Cut the edges to make one large square, then divide it into four parts.
Scoop about a tablespoonful of the filling and place it in the middle. Fold the dough over the filling and seal using a fork and brush with egg whites.
In a pan over medium heat deep-fry the pies for 2-3 minutes on each side until golden brown. Remove from the pan and drain excess oil. Allow it to cool for about five minutes before serving.
Serving Size Makes 8-10 pies