Step 1

Preheat oven to 350 degrees F (175 degrees C), or 325 degrees F (165 degrees C) for coated pans. Spray two 8-inch round cake pans with cooking spray.

Step 2

Stir sugar, frozen coconut, and sour cream for filling together in a medium bowl until well combined. Set aside in the fridge until ready to use.

Step 3

Combine cake mix, pudding mix, water, oil, eggs, egg whites, and vanilla for cake in the bowl of a stand mixer fitted with a whisk attachment. Blend on medium speed for 3 minutes, scraping down the sides of the bowl as necessary. Pour batter into the prepared pans.

Step 4

Bake in the preheated oven until golden brown and a toothpick inserted into the center comes out clean, about 25 minutes. Remove from the oven and cool in the pans for 5 minutes, then invert onto a parchment-lined cooling rack to cool completely, about 30 minutes.

Step 5

Use a large, serrated knife to cut each cake layer in half lengthwise; you will have 4 pieces. Flip top pieces so all cut sides are facing up. Stack layers and filling on a serving platter as follows: 1 bottom layer, 1/3 of the filling, 1 top layer, 1/3 of the filling, remaining bottom layer, remaining filling, remaining top layer. Cover and chill, 30 minutes to overnight.

Step 6

Combine sugar, corn syrup, egg whites, water, vanilla, and cream of tartar for frosting in a large heat-proof bowl; stir until well combined. Place the bowl over a double boiler and whisk using an electric hand mixer until thick and glossy, about 7 minutes. Take care that the double boiler doesn't reach higher than a slow simmer.

Step 7

Spread frosting over cooled cake immediately and sprinkle with shredded coconut.