Using an electric mixer on medium speed, cream the butter until it's smooth (about 1 minute).  

Add in the sugar and continue to mix for another minute, until the mixture is light and fluffy.  

Add in the eggs and vanilla extract and continue to mix until well combined.  Scrape down the sides of the bowl as needed.

In a medium bowl, sift together the flour, salt, baking powder, baking soda and cornstarch.

Slowly add the dry ingredients to the wet ingredients and mix until just combined.  

Add in the white chocolate chips and sprinkles and either briefly use the electric mixer, or mix by hand.  

Spread the cookie dough evenly in the prepared baking pan and bake for 25-30 minutes, or until very lightly browned.  Allow the cake to cool in the pan for 10-15 mins and then transfer to a wire rack to cool completely.  Decorate the cake with the whipped vanilla frosting once it has completely cooled.

Place the butter in the bowl of an electric mixer fitted with the whisk attachment.

Whip on medium-high speed until smooth, about 1 minute.

Add the confectioners’ sugar to the bowl and mix on medium-low speed just until incorporated.

Add in the salt and continue to beat on medium-high speed until smooth, about 1-2 minutes.

Mix in the heavy cream and vanilla on low speed just until incorporated.

Increase the mixer speed and whip on high speed until light and fluffy, scraping down the bowl as needed, about 4-5 minutes.

*You can either decorate your cake with white frosting, or if you want to dye it like I did, divide the frosting into 4 bowls.  Dye one bowl pink, one blue and one yellow - leave the last one white.  Place each color into its own piping bag.  Then add your desired extra large frosting tip to a totally different bag.  Cut the tips of the frosting bags and place them all into the bag with the tip.  Now you will have perfectly color swirled frosting!