Preheat the oven to 350 degrees F.

Line a 9" square pan with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar, and salt on medium speed until light & fluffy, about 2-3 minutes.

Add the egg, egg yolk, vanilla, and oil, and mix until well combined.

Add the flour and cocoa powder, and mix on low speed until fully combined.

Transfer the cookie dough to the prepared square pan, and use an offset spatula to spread it into an even layer.

Bake the sugar cookie bars for about 25 minutes or until they appear set.

Allow the sugar cookie bars to cool in the pan completely.

In a medium saucepan, melt the butter over low-medium heat.

Once melted, turn the heat off, then add the chocolate, heavy cream, powdered sugar, cocoa powder, salt, and vanilla. Stir constantly until everything is melted and fully combined.

Pour the mixture through a fine mesh sieve into a medium bowl. Place in the fridge to cool, stirring every 10 minutes or so until it reaches a spreadable consistency. Leave at room temperature until ready to use.

Spread the fudge frosting onto the cooled sugar cookie bars, then top with chocolate sprinkles.

Cut into 16 squares, and enjoy!