Heat the oven to 350°F (175°C). Grease and flour one 9 x 13-inch pan, or grease and line two 9-inch round cake pans with parchment.

Combine the lemon cake mix and lime gelatin in a large mixing bowl. Add the eggs, vegetable oil, orange juice, key lime juice, and optional lime zest, then beat for about 2 minutes until smooth and well blended.

Pour the batter into the prepared 9 x 13-inch pan, or divide it evenly between the two round pans. Bake the sheet cake for 25 to 30 minutes, or bake round layers for about 20 to 25 minutes.

Check for doneness when the center springs back lightly and a toothpick comes out clean or with a few moist crumbs. Cool the cake in the pan for 10 minutes, then transfer it to a cooling rack or leave it in the pan until completely cool.

For the frosting, beat the softened butter and cream cheese until smooth. Gradually add the confectioners’ sugar, then beat in the key lime juice and optional zest until creamy.

Spread the frosting over the completely cooled sheet cake, or frost between the round layers and over the top and sides. Chill the cooled cake for 20 to 30 minutes first if the frosting feels difficult to spread.

For the optional glaze, whisk 1/2 cup key lime juice with 1/2 cup confectioners’ sugar and spoon or brush it over the warm cake before cooling and frosting. Slice the finished cake and garnish with lime zest or thin lime slices if desired.