Combine the wet and dry ingredients and mix until all of the ingredients create a smooth batter.
Spread the batter evenly in a 9x13 prepared pan. Bake at 350F for 18 to 20 minutes, or until the cake is lightly golden and a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 15 minutes.
Add the softened butter to a large mixing bowl and beat until smooth and creamy. Gradually add the powdered sugar, mixing on low speed until incorporated. Add the vanilla extract, salt, and 2 tablespoons of heavy cream or milk. Beat for another 2 to 3 minutes, or until the frosting is light and fluffy.
Choose a clean, straight-sided drinking cup with a rim approximately 2 1/2 to 3 inches wide. Press the rim firmly into the sheet cake to cut out 8 round cake layers.
Place one cake round inside the same cup used to cut the layers. Spread a thin, even layer of buttercream over the cake. Place a second cake round on top and gently press it into place. Spread another layer of buttercream over the top. Repeat with the remaining cake rounds.
Pour the round sprinkles into a shallow bowl or plate. Gently press the frosted top of each mini cake into the sprinkles.