In a large bowl, beat together the butter and sugar until smooth. Add the sweetened condensed milk and vanilla extract and beat until combined.
Add the flour and mix well. Fold in the mini chocolate chips and sprinkles.
Line an 8-inch x 8-inch baking pan with parchment paper. Then press the cookie dough into an even layer in the pan. (The batter will seem a bit sticky before it has set but once it spends time in the fridge it has the perfect cookie dough texture, even when fridge-cold.
Melt the milk chocolate chips and shortening together in a small saucepan over low heat, stirring until smooth.
Pour the melted chocolate over the cookie dough and spread it into an even layer. Add more sprinkles on top if desired.
Chill for at 3 hours in the fridge.
Use the parchment paper to lift the cookie dough block from the pan and then cut it into 12 squares. Serve cold. Stor any leftovers in the fridge for up to 5 days in an airtight container.
Notes
Heat-treated flour is recommended to remove the raw flour taste.
These are so tasty but very sweet. Twelve bars doesn't seem like too big a yield but I was happy with half a bar as a serving at one time. So feel free to cut the block into smaller pieces.