Preheat the oven to 190C/375F/Fan 170C/Gas Mark 5 and line 2 baking sheets with baking paper. Using an electric mixer or wooden spoon, beat the butter and sugar together until soft and creamy.
Beat in the egg and vanilla extract. Sift in the flour, baking powder and salt. Add the chopped pecans and mix to form a firm dough.
Roll the dough into 16 even-sized balls and place on the baking sheets allowing space between each one for them to spread. Flatten slightly with the back of a fork and place a pecan half on the top of each one. Bake for 15 mins until golden.
Leave on the baking sheets for 5 mins before transferring to a wire rack to cool. As they cool the cookies will become firm and crisp. Store in an airtight container.
Notes
Gluten-free flour sometimes absorbs more liquid than ordinary flour, so if the mixture is too crumbly to form a dough add a little milk or water and mix until the dough comes together in a ball.