Creating the Perfect Vanilla-Chocolate-Strawberry Sheet Cake is straightforward if you follow these steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.

2. Mix Dry Ingredients: In a large bowl, sift together the flour, granulated sugar, cocoa powder, baking powder, and salt.

3. Combine Wet Ingredients: In another bowl, whisk together the milk, vegetable oil, eggs, and vanilla extract until well combined.

4. Combine Mixtures: Gradually add the wet ingredients to the dry ingredients, mixing until smooth and no lumps remain.

5. Divide Batter: Evenly divide the batter into two bowls. Add the pureed strawberries to one bowl and mix until fully incorporated.

6. Layer the Cake: Pour the chocolate batter into the prepared baking pan, smoothing the top. Then, carefully layer the strawberry batter on top of the chocolate batter.

7. Bake: Place in the preheated oven and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

8. Cool: Remove the cake from the oven and allow it to cool in the pan for 20-30 minutes. Then, transfer to a wire rack to cool completely.

9. Prepare the Frosting: In a mixing bowl, beat the softened butter until creamy. Gradually add powdered sugar, vanilla extract, and milk until you achieve a smooth, spreadable consistency.

10. Frost the Cake: Once the cake is completely cool, spread the frosting evenly over the top. You can leave it as is or create decorative swirls.