Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter and brown sugar until smooth. Add the eggs and vanilla extract, mixing well until fully incorporated.
Sprinkle the salt and all-purpose flour over the wet ingredients. Gently fold in the flour using a spatula until just combined. Do not overmix! Finally, fold in the large rainbow sprinkles, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, smoothing the top with a spatula. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the blondies to cool completely in the pan.
While the blondies are cooling, prepare the frosting. In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, mixing until fully combined. Add the vanilla extract and 2 tablespoons of milk, mixing until smooth. If the frosting is too thick, add an additional tablespoon of milk to reach your desired consistency.
Once the blondies are completely cool, spread the frosting evenly over the top using a spatula. Finish with a sprinkle of rainbow sprinkles on top for that extra festive touch.
