1. Prepare the Cake: Preheat your oven to 450°F (230°C). Line a 17×12 inch pan with parchment paper to prevent sticking. In a mixing bowl, whip together 2 eggs, 3 yolks, and ½ cup sugar until the mixture becomes thick and pale. Stir in 1 tsp vanilla extract, and gently fold in ⅓ cup cake flour, 3 tbsp cornstarch, and ¼ cup red & green Wilton batter bits until just combined.
2. Make the Meringue: In a separate bowl, beat the 2 egg whites with 1 tbsp sugar until stiff peaks form. Gently fold the meringue into the yolk mixture. This step is crucial for achieving a light and fluffy texture in your cake.
3. Bake: Carefully spread the batter evenly in the prepared pan and bake for 6-7 minutes or until the cake turns golden brown. Once baked, take the cake out of the oven and immediately roll it up in a clean kitchen towel dusted with powdered sugar. Let it cool completely while rolled up, as this helps it maintain its shape later.
4. Prepare the Filling: While your cake cools, prepare the filling by creaming 1 cup softened butter in a mixing bowl until light and fluffy. Gradually mix in 3 cups powdered sugar and 1 tsp vanilla extract, adjusting the consistency with 1-3 tbsp heavy cream as needed. Once the filling is creamy, fold in 3-4 tbsp of red & green sprinkles for a festive touch.
5. Assemble: Once the cake has completely cooled, gently unroll it, being careful not to crack the edges. Spread the vanilla buttercream filling evenly over the surface of the cake, then carefully roll the cake back up, starting from one end. Wrap it in plastic wrap and chill in the refrigerator for about 30 minutes to help set the shape.
6. Decorate: To finish, melt 4 oz of white chocolate in a microwave or over a double boiler until smooth. Drizzle the melted chocolate over the chilled cake roll and immediately sprinkle with more red and green sprinkles. Allow the chocolate to set before slicing for serving.

By following these steps, you will create a beautifully rolled cake that is not only delicious but also a feast for the eyes.