Preheat oven to 325°. Lightly spray a 12-inch cast-iron skillet with baking spray with flour.
In a large bowl, beat cake mix, 3 eggs, ½ cup lime juice, ½ cup water, oil, and gelatin with a mixer at medium speed until thick and smooth, about 2 minutes, stopping to scrape sides of bowl. Spread batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool on a wire rack for 5 minutes. Using the handle of a wooden spoon, poke holes in warm cake at 1-inch intervals.
Meanwhile, in a small enamel-coated cast-iron Dutch oven, whisk together ¼ cup granulated sugar, 2 teaspoons zest, and remaining ½ cup lime juice. Cook over medium-low heat, whisking constantly, until steaming (do not boil).
In a medium bowl, whisk together remaining 3 eggs and remaining ¼ cup granulated sugar. Gradually whisk hot lime mixture into egg mixture. Pour egg mixture into pan, and cook over medium-low heat, whisking constantly, until mixture is thick enough to coat the back of a spoon, 5 to 10 minutes. Strain mixture through a fi ne-mesh sieve into a medium bowl, discarding solids. Gradually stir in butter until melted and well combined.
Spread lime mixture onto warm cake. Gently tap pan on counter 4 to 5 times to release any air bubbles. Cover with plastic wrap, pressing directly on lime mixture to prevent a skin from forming. Let cake stand for 30 minutes, and then refrigerate until cold, at least 4 hours or up to 3 days.
Just before serving, in a large bowl, beat cream cheese and confectioners’ sugar with a mixer at medium-low speed until smooth and creamy, stopping to scrape sides of bowl. Beat in remaining 2 teaspoons zest. Gradually add cream until combined, stopping to scrape sides of bowl; gradually increase mixer speed to medium-high, beating until stiff peaks form, about 2 minutes. Spread onto cake. Garnish with lime and graham cracker pieces, if desired.
Notes
*We used Betty Crocker White Cake Mix and Nellie and Joe’s Famous Key West Lime Juice.
Recipe by Southern Cast Iron at https://southerncastiron.com/key-lime-poke-cake/