Preheat oven to 350 degrees F
Line 8x8 pan with parchment, set aside.
SUGAR COOKIE:
Place flour, baking soda, salt in bowl, mix to combine, set aside.
In bowl of mixer fitted with paddle attachment cream the butter.
Add in the sugar and mix until pale and light.
Add in the vanilla, continue to mix.
Add in the eggs, continue mixing.
With mixer on low add in the flour mixture. Mix until completely combined and dough forms.
Scoop dough into pan, with floured hands press dough evenly around the bottom of the pan until evenly covered.
Bake 12-14 minutes or until lightly golden.
Cool completely.
BUTTERCREAM:
In medium sauce pan add the egg whites and sugar.
On medium heat you are going to whisk egg mixture until sugar is completely dissolved (you can test it by rubbing a small amount between your fingers to test it, it should be completely smooth).
Transfer to mixing bowl and with whisk attachment whisk on high until stiff peaks form and mixture is cooled.
On low speed add the butter one piece at a time. Mix until well combined.
Switch to paddle attachment, add vanilla and vanilla beans, mix until smooth and glossy.
ASSEMBLY:
Once base is completely cooled and buttercream has been made. Scoop the buttercream onto the base and spread around with an off set spatula. Sprinkle with sprinkles.
Tip: you can cut right away but for a cleaner cut refrigerate 10-15 minutes so the buttercream firms up a bit.