Preheat your oven to 350°F (175°C) and line a 13×9-inch baking pan with parchment paper, leaving overhang on the sides for easy removal.
In a large bowl, use an electric mixer to beat the softened butter and granulated sugar on medium-high speed for 2-3 minutes until light and fluffy.
Add eggs one at a time, beating well after each addition. Mix in the vanilla extract until just combined.
In a separate bowl, whisk together the flour, salt, and baking powder.
Gradually add the flour mixture to the wet ingredients, mixing on low speed just until combined. Don't overmix.
Transfer the dough to your prepared baking pan and use lightly floured hands or the back of a measuring cup to press it into an even layer.
Bake for 20-25 minutes until the edges are lightly golden and the center appears set but still soft.
Allow the bars to cool completely in the pan before frosting, at least 30 minutes.
For the frosting, beat the softened butter until creamy. Gradually add powdered sugar, alternating with milk, until the frosting reaches a smooth, spreadable consistency. Beat in vanilla extract, almond extract if using, and salt. Add food coloring if desired.
Spread the frosting evenly over the cooled bars and decorate with sprinkles if using.
For the cleanest cuts, chill the frosted bars for 15-20 minutes before cutting. Lift the entire slab out using the parchment paper overhang, then use a sharp knife to cut into squares or rectangles.