In a medium saucepan, whisk together condensed milk, Key lime juice, yolks, and salt. Cook over medium heat, whisking constantly, until thickened, 6 to 8 minutes. Strain condensed milk mixture through sieve. Whisk in zest and ¼ teaspoon vanilla. Place a sheet of plastic wrap directly on surface and refrigerate until chilled, about 1 hour.
Fill a medium Dutch oven with 2 inches of oil; heat over medium heat until a deep-fry thermometer registers 365°. Place a wire rack over a rimmed baking sheet.
Separate biscuit dough into8 rounds. Let stand for 10 minutes at room temperature. (This step allows the biscuits to rise slightly and form a better round shape.) Working in batches of 2 to 4, fry until golden brown, about 3 minutes, turning often. Using a spider strainer, transfer doughnuts to prepared wire rack and let cool completely.
Using a small knife, cut a hole in the side of each doughnut and hollow inside using a thin wooden spoon handle.
Spoon condensed milk mixture into a piping bag fitted with a ¼-inch round tip; pipe evenly into each doughnut.
In a medium shallow bowl, stir together confectioners’ sugar, milk, corn syrup, butter, and remaining ½ teaspoon vanilla until smooth and well combined. Working one at a time, dip doughnuts halfway into glaze; let excess drip off. Place, icing-side up, on wire rack. Sprinkle with graham crackers and zest, if desired. Serve immediately or let stand until set, about 20 minutes.
Notes
*We used Pillsbury Grands! Southern Homestyle Original Biscuits.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/key-lime-filled-doughnuts/