Preheat your oven to 325°F (163°C). In a medium bowl, mix together graham cracker powder and melted unsalted butter until crumbly. Press this mixture firmly into the bottom of a greased 9x13-inch pan, creating an even layer. Bake for 10 minutes, then remove from the oven and allow it to cool while you prepare the cake batter.

In a large mixing bowl, whisk together buttermilk, sour cream, egg whites, vegetable oil, vanilla extract, lime juice, and lime zest until fully combined. In another bowl, sift the white cake mix to prevent lumps, then gently fold it into the wet mixture. Be careful not to overmix.

Pour the cake batter evenly over the pre-baked graham cracker crust in the pan. Bake in your preheated oven for 27–30 minutes, or until the cake is set and springs back lightly when touched. Let it cool completely in the pan.

In a mixing bowl, whip the room temperature unsalted butter with an electric mixer until it becomes fluffy. Add in salt, vanilla extract, lime juice, and heavy cream. Gradually mix in powdered sugar, adjusting until you reach your desired frosting consistency.

Once the cake has cooled completely, use an offset spatula to spread the fluffy key lime buttercream evenly over the top. Sprinkle graham cracker crumbs over the buttercream, and then garnish with fresh key lime slices.