Preheat the oven to 350°F. Spray a 13- x 9-inch baking pan with cooking spray.

Mix the cake batter by beating the cake mix, sparkling water, canola oil, eggs, 1 package of the pistachio pudding mix, and 1/2 teaspoon of kosher salt in a large bowl with an electric mixer on medium speed until light and fluffy, about 2 minutes.

Carefully fold in the sweetened coconut, toasted pecans, and chopped pistachios.

Pour the batter into the prepared pan and spread it out in an even layer.

Bake until the top is lightly golden and a wooden pick inserted in the center comes out clean, about 25 to 30 minutes.

Allow the cake to cool completely in the pan on a wire rack, about 1 1/2 hours.

Whip the heavy cream until it just starts to thicken, about 3 minutes. Add the powdered sugar, 3 tablespoons of the remaining pistachio pudding mix, and the remaining 1/2 teaspoon of salt. Continue beating until thickened and smooth.

Spread the frosting onto the cooled cake.

Garnish with additional flaked coconut and chopped pistachios.