Preheat your oven to 350°F (175°C) and prepare four 9-inch round cake pans by greasing them and lightly dusting with flour. Set aside.

In a medium mixing bowl, whisk together the flour, salt, baking powder, and baking soda until well combined, then set the dry mixture aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed until smooth and creamy, about 1 minute. Add the sugar and continue beating on high speed for about 5 minutes, pausing occasionally to scrape down the sides of the bowl with a spatula, until the mixture is light and fully blended. Reduce the speed to medium-high and add the eggs one at a time, beating well after each addition. Stir in the vanilla extract until incorporated.

Gradually add the dry flour mixture to the butter mixture in three additions, alternating with the buttermilk. Mix just until the batter is smooth and slightly thick, being careful not to overmix.

Divide the batter evenly into four separate bowls. Tint each portion with one of the following gel food colorings: green, blue, purple, and pink (optional), stirring until each is fully blended and the colors are vibrant. Pour each colored batter into the prepared cake pans.

Bake the cakes in the preheated oven for approximately 25 minutes, or until a toothpick inserted in the center comes out clean. Remove the pans from the oven and place them on wire racks to cool completely, which should take about 25 minutes.

To prepare the frosting, use a clean bowl and paddle attachment in your stand mixer. Beat the butter and cream cheese together on medium speed until creamy, about 2 minutes. Add the confectioners' sugar, heavy cream, and vanilla extract. Begin mixing on low speed for 30 to 60 seconds to avoid sugar spillage. Increase the speed to high and continue beating for about 3 minutes, scraping the bowl as needed, until the frosting is light and fluffy. Stir in the salt.

Once the cake layers have cooled, use a 4-inch round cookie cutter to carefully cut out the center of two of the layers, creating cake rings.

To assemble the cake, place one whole (uncut) cake layer on a cake stand or serving plate. Spread a layer of frosting evenly over the surface using an offset spatula or butter knife. Place one of the cake rings on top and spread more frosting across its surface and inside the center hole. Repeat the process with the second cake ring, stacking it on top and frosting as before. Fill the hollow cavity in the center with assorted candies and sprinkles until full. Finish by placing the second whole cake layer on top, covering the candy-filled center.

Frost the top and sides of the cake evenly with the remaining frosting. If desired, tint some of the leftover frosting with a drop of pink gel food coloring and pipe decorative accents around the cake using a piping bag.

When ready to serve, cut the cake into slices and enjoy the surprise of candies spilling out onto the plate.