Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan and line it with parchment paper or lightly coat it with non-stick cooking spray.

Prepare the Cake Batter: In a medium bowl, whisk together the flour, baking soda, and salt. In a separate large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the egg yolks, one at a time, followed by the vanilla extract.

Alternate Mixing: Gradually add the dry ingredients to the creamed mixture, alternating with the buttermilk. Mix until just combined. Stir in the shredded coconut and chopped pecans or walnuts.

Whip Egg Whites: In another clean mixing bowl, beat the egg whites until stiff peaks form. Gently fold the beaten egg whites into the cake batter until well combined.

Bake the Cake: Pour the batter into the prepared baking pan and spread it out evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake from the oven and let it cool completely in the pan on a wire rack.

Make the Cream Cheese Frosting: In a mixing bowl, beat together the softened butter and cream cheese until smooth and creamy. Gradually add the powdered sugar, one cup at a time, beating well after each addition. Stir in the vanilla extract until incorporated.

Frost the Cake: Once the cake has cooled completely, spread the cream cheese frosting evenly over the top. If desired, sprinkle additional shredded coconut and chopped pecans or walnuts on top for garnish.

Serve and Enjoy: Cut the Italian Cream Sheet Cake into squares and serve. Store any leftovers in an airtight container in the refrigerator.