Preheat your oven: Start by preheating the oven to 350°F (175°C). Prepare a 15x10-inch jelly roll pan by greasing it or lining it with parchment paper to prevent sticking.
Beat the eggs and sugar: In a large mixing bowl, beat 4 large eggs and 1/2 cup sugar together until frothy and considerably lighter in color. This process usually takes about 4-5 minutes.
Add wet ingredients: Gradually mix in 1/4 cup vegetable oil, 1/2 cup buttermilk, and 1 tsp vanilla extract until well combined.
Sift dry ingredients: In another bowl, sift together 1 cup all-purpose flour, 1/2 cup unsweetened cocoa powder, 1 tsp baking powder, 1/2 tsp baking soda, and 1/4 tsp salt. This step helps to remove any lumps and evenly distribute the leavening agents.
Combine wet and dry ingredients: Carefully fold the dry ingredients into the wet mix, being careful not to over-mix, which could result in a dense cake.
Add food coloring: Add purple food coloring to the batter in small amounts, mixing until you achieve the desired shade.
Pour batter into the pan: Once everything is combined, pour the batter into the prepared jelly roll pan. Bake in the preheated oven for 12-15 minutes or until the cake is springy to the touch and a toothpick comes out clean.
Prepare the filling: While the cake is baking, whip up the filling by mixing 1 cup softened cream cheese, 1/2 cup powdered sugar, and 1 tsp vanilla extract until creamy and smooth.
Roll the cake: Once baked, immediately turn the warm cake onto a clean towel that has been dusted with powdered sugar. Gently roll the cake with the towel from the short end, then allow it to cool completely.
Spread the filling: Once the cake has cooled, carefully unroll it, spread the cream cheese filling evenly over the surface, and then roll it back up with the seam side down.
Optional topping: If you'd like to add a topping, whip 1 cup heavy whipping cream until stiff peaks form and spread it on the outside of the cake.
Make sure to allow the cake to cool completely before adding the filling to avoid melting the cream cheese. You can garnish with edible flowers or colorful sprinkles for a delightful touch!