In a large mixing bowl, cream the butter and sugar together until light and fluffy.
Add the egg and mix until smooth, then add the yolk and mix until smooth.
Add the vanilla and mix again until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, and salt.
Add the dry ingredients to the wet mixture and mix until just combined. Do not overmix.
Cover the bowl with plastic wrap and chill the dough in the fridge for 30 minutes before portioning.
Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Bring the dough out of the fridge and dump it onto a lightly floured counter and bring it together gently with your hands.
Divide the dough into 16 portions of about 40 grams each. Shape each piece as desired to match your mold — typically into balls, squares or rectangles.
Lightly flour each mold by adding a small amount of flour and moving the mold around so every crevice is evenly coated. Tap the mold gently on the counter to remove any excess flour.
Place one portion of dough onto the mold and press it gently, starting from the center and moving toward the edges, making sure the dough fills all the details.
Use your fingers to gently lift around the edges
Turn the mold upside down and lightly tap it on your hand or on the counter to release the cookie.
Transfer the shaped cookie to the prepared baking sheet.
Bake for 10 to 12 minutes, or until the edges are lightly golden and the tops are set.
Let cool on the baking sheet for 5 minutes before adding any toppings and wait 10 minutes before transferring to a wire rack to cool completely.