Preheat oven to 325F and line an 8x8 metal baking pan with parchment paper leaving the sides high to pull out easier once baked.

In a medium bowl whisk together all-purpose flour, sea salt and baking powder. Set aside.

Using a stand mixer, mix sugar and melted butter on low speed to combine.

Add in egg yolk and pure vanilla. Mix again on low for about 30 seconds, scrape down the sides as you go.

Transfer dry ingredients and run on low until the dough is fully combined (about 60 seconds).

Bake for about 20 minutes, the edges should look set, and a toothpick will come out of the center with moist crumbs. Cool on a wire rack fully then start your frosting.

Combine icing sugar, butter and milk in the bowl of a stand mixer until smooth using the paddle attachment (about 2-3 minutes).

Place dollops of frosting over cooled bars (I prefer to do this in the pan for cleaner edges), then add any sprinkles if desired.

Again, scrape down the sides as needed. With an offset spatula (or metal spoon) flatten cookie dough into the prepared pan.

Slice and serve or chill for cleaner cuts.