Preheat your oven to 350°F (175°C). Grease the 13x9-inch baking pan or line it with parchment paper for easy removal.
In a mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In another large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy (about 2-3 minutes).
Beat in the egg and both extracts (vanilla and almond) until well combined.
Gradually add the flour mixture into the wet ingredients, mixing until just combined. Be careful not to overmix.
Spread the cookie dough evenly into the prepared baking pan, smoothing the top with a rubber spatula.
Bake in the preheated oven for 20-25 minutes, or until the edges start to turn golden and a toothpick inserted in the center comes out clean.
Allow the sugar cookie bars to cool completely in the pan on a cooling rack before icing.
For the icing, whisk together the powdered sugar and milk in a small bowl until smooth and creamy. Adjust the consistency with more milk or powdered sugar as needed.
Once cooled, spread the icing evenly over the bars and sprinkle with decorations if desired.
Cut into 12 bars and serve.