Place butter and sugars in a large mixing bowl. Beat with an electric mixer on medium to medium-high speed until mixture is creamy, scrapping down the sides of the bowl with a rubber spatula occasionally, about 5 to 7 minutes.
Add in egg and vanilla and beat on medium speed for 1 minute until combined.
Add in flour, ground almond, baking powder and salt and beat on low speed until just combined. Alternatively, use a rubber spatula and stir until the cookie dough comes together.
Scoop out the dough onto a large piece of clingfilm and pat it out into a disc of about 1-inch thick. Wrap up the dough with the clingfilm and chill in the refrigerator for at least 1 hour.
Preheat the oven to 180°C / 355°F. Line a baking sheet with baking parchment.
Unwrap the dough and place it on a large piece of clingfilm. Place the clingfilm used to wrap the dough on top of the dough to prevent sticking. Use a rolling pin or a wine glass bottle and roll out the dough until it's 5-mm (1/4-inch) thick. If the dough feels too soft, chill it in the refrigerator for another 15 minutes before cutting the shapes out.
Dip a cookie cutter into flour and cut out a cookie shape, then transfer it onto the baking sheet. Repeat, combining and rolling out the scraps, until all the cookie dough has been used up. If the dough is becoming too sticky, chill it again before using.
Bake the cookies for 10 to 12 minutes until lightly golden around the edges. Cool the cookies for 3 minutes on the baking sheet, then transfer to a rack to cool completely.
Once the cookies have cooled completely, decorate the with royal icing or melted chocolate.
NOTES
To decorate the Christmas Tree Cookies with royal icing like I did: Pipe a green border icing around the cookie and flood it with green icing. Set aside for 5 to 10 minutes so that green icing set slightly (if you do not let it set slightly, the other colours will bleed into the green icing). Pipe in small dots of red, blue, yellow, white (or any other colours) into the green icing to mimic the Christmas ball ornaments.
It's best to set the royal icing / chocolate overnight before storing them in cookie jars or wrapping them up in small gift bags.