Preheat oven to 180ºC (350ºF). Line a large baking tray with parchment paper. Set aside.
Cream the butter: In a large mixing bowl, use electric hand beaters, or a stand mixer with the paddle attachment, add the softened butter and sugar. Beat on medium-high speed for 1 minute until pale and creamy. Scraping down the sides as needed.
Add the egg: Beat in the egg until incorporated, about 10 seconds.
Incorporate dry ingredients: Sift over flour and baking powder. Beat on low speed until the mixture begins to form a soft, crumbly dough.
Form the dough: Turn off the beaters. Using your hands or a wooden spoon, bring the dough together.
Roll out the dough: Lightly flour your benchtop or a large board. Transfer the dough onto the floured surface and lightly knead. Dust a rolling pin with flour and roll out the dough to 4-5mm thickness.
Cut out biscuits: Using a rectangle cookie cutter measuring 6cm length x 4cm width (2 inches x 1.5 inch) or a pizza/pastry wheel and cut out the biscuits. Place on the lined baking sheet, leaving a small gap between each. Repeat with the remaining dough, re-rolling any excess.
Bake: Bake for 15-20 minutes or until lightly golden around the edges. Start checking at 15 minutes. Allow to cool on the sheet for 10 minutes, then transfer to a wire rack to cool completely.