Step 1: Preheat the oven to 350°F and grease a 9x13-inch baking dish with non-stick spray. Sift the cake flour, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer or use a hand-held mixer. With the paddle attachment, beat the ingredients together on low speed for a few seconds to gently combine. Add the butter, vanilla, and ½ cup of milk. Mix on medium speed until the dry ingredients are moistened, about 1 minute. The batter will be thick at this point.
Step 2: With the paddle attachment, beat the ingredients together on low speed for a few seconds to gently combine. Add the butter, vanilla, and ½ cup of milk. Mix on medium speed until the dry ingredients are moistened, about 1 minute. The batter will be thick at this point.
Step 3: Beat the remaining milk and sour cream into the mixing bowl at a medium speed. Add in the eggs one at a time mixing for a few seconds after each addition. Scrape down the sides and up the bottom of the bowl and mix on low making sure everything is completely combined. Don't overmix cake batter or it will lead to a tougher crumb.
Step 4: Divide the batter into equal portions in separate bowls (as many bowls as you'd like colors) and add different colors of food coloring to each.
Step 5: Spoon the colored batters into the cake pan, swirling slightly for a marbled effect if desired. Bake for 30-35 minutes until a toothpick comes out clean when inserted in the center of the cake.
Step 6: Using the end of a wooden spoon or a large straw, poke holes all over the cake. Space them about 1 inch apart.
Step 7: In a large mixing bowl, whisk together the instant pudding and cold milk until it thickens slightly, approximately 3-4 minutes.
Step 8: Pour the pudding mixture over the cake. Use a spatula to gently spread the pudding, ensuring it fills the holes of the cake. Cover with plastic wrap and refrigerate for at least 4 hours to overnight to allow the pudding to set.
Step 9: If using heavy cream: In the bowl of a stand mixer, beat the heavy cream, powdered sugar, and vanilla extract until stiff peaks form. If using Cool Whip: Simply allow it to thaw according to package instructions. Spread your whipped topping evenly over the top of the cooled cake. Decorate with your favorite Easter sprinkles. Slice, serve, and enjoy!