Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving a little overhang for easy lifting, or lightly grease the pan.

Beat the softened butter and granulated sugar for 2–3 minutes, until light, creamy, and noticeably paler. Add the eggs one at a time, mixing after each addition, then blend in the vanilla extract.

Whisk the flour, baking powder, and salt together in a separate bowl. Gradually add the dry mixture to the butter mixture and mix only until no obvious streaks of flour remain.

Press the thick dough evenly into the prepared pan using lightly greased hands or an offset spatula. Bake for 15–20 minutes, until the edges are just lightly golden and the center looks set but still soft.

Cool the bars completely in the pan before frosting, allowing approximately 45–60 minutes for cooling and finishing. For the frosting, beat the butter and cream cheese until smooth, gradually add the powdered sugar, and mix in the vanilla.

Add milk or cream a little at a time until the frosting reaches a smooth, spreadable consistency. Spread it over the cooled bars, decorate with sprinkles if desired, and cut into approximately 24 squares.