Preheat your oven to 350°F (175°C).
In a large mixing bowl, cream together the 3/4 cup of unsalted butter and 1 cup of granulated sugar using an electric mixer until light and fluffy, about 2-3 minutes.
Add the 2 large eggs and 1 teaspoon of vanilla extract to the mixture and beat until well combined.
In a separate bowl, whisk together 2 1/4 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Pour the cookie batter into a greased 9x13 inch baking pan and spread it evenly with a rubber spatula.
Bake for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Remove the pan from the oven and let the bars cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
In a medium bowl, beat together 8 tablespoons of room temperature butter, 3 cups of powdered sugar, 1/4 teaspoon of vanilla extract, and 3 tablespoons of milk until smooth and creamy.
Spread the frosting evenly over the cooled bars.
Decorate the frosted bars with sprinkles, M&M's, and white chocolate chips.
