Preheat the oven to 350°Fahrenheit and line two baking sheets with parchment paper. Set aside.
In a medium bowl, combine the flour, baking powder, baking soda and salt.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, combine the butter and sugar together. Mix until light and fluffy.
Add the egg and almond extract, and mix until combined and it turns light in color.
Add the dry ingredients and mix until just combined.
Using a cookie scoop or spoon, scoop out 1 ½ tablespoon sized cookie dough balls onto your prepared baking sheets. You want to make sure you have 4-5 inches between each cookie dough ball.
Bake one tray at a time for 8-11 minutes. The edges should have set and the middles will still look soft. Let them cool on the baking sheet for 10 minutes before transferring to a wire cooling rack to cool completely.
Once the cookies have cooled completely you can make the buttercream. In a medium bowl, combine the butter with the almond extract, salt and powdered sugar. Feel free to add more powdered sugar until it is the consistency you like.
Frost the cookies with the buttercream and decorate with sliced almonds!