In a large bowl, cream butter with sugar until light and fluffy. Whisk in the egg, egg yolk, vanilla extract and salt.
In a smaller bowl, mix flour with baking powder. Add flour to the butter mixture in 2-3 additions, whisking well after each addition, until you get a smooth dough. If the dough is too dry and crumbly, add a tablespoon of milk until you get a kneadable dough.
Transfer the dough to a lightly floured working surface and knead it for a minute, making sure it’s smooth and compact. If the dough is sticky, add more flour. (The dough should be soft enough for you to make an indentation in it with your finger, but not sticky.)
Flatten dough into a disk, wrap in plastic wrap and chill in the refrigerator for at least 30 minutes. Set the oven to 350°F (175°C) and line two baking sheets with parchment paper. Set aside.
Take out the dough and divide it into 3 equal parts. Set one part aside. Take the other two and make 23 smaller lumps of dough.
Roll each lump into a smooth ball. Place dough balls on a baking sheet about 1 inch (2.5 cm) apart. Gently press each one into the baking sheet to flatten the bottom.
Split the remaining 1/3 portion of dough into 23 equal parts and roll each one into a log, about 2 1/2 inches (6 cm) long. Place them on the other baking sheet, about 1 inch (2.5 cm) apart and set it aside.
Bake cookies in the center of the oven for 10-15 minutes, until they rise and get slightly golden brown around the edges (the tops will stay pale).
As you take out the first cookie sheet (the “mushroom caps”), put the second one with the “stems” in the oven and bake for 10-15 minutes (until golden brown on the bottom).
Immediately start prepping the first batch of cookies, while they are still warm (this is important!). Cut a circle in the bottom of each cookie and scoop out about 2 teaspoons of crumbs. Do this with all the cookies and set the crumbs aside.