Preheat the oven to 350°F(177C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Toast the sesame seeds in a small pan at low-medium heat and stir frequently until they are golden brown and give off their fragrance. Remove the sesame seeds from the pan at this stage as they can pop and jump out of the pan like popcorns. They can also easily burn with the heat of the pan so place the sesame seeds into a bowl and set aside.
Combine the flour and baking powder in a bowl and set aside.
Place the butter in a mixing bowl and beat on medium speed for 1 to 2 minutes until it is soft with a hand-held mixer or with paddle attachment if using a stand-mixer. Add the sugar and mix until creamy. Add the egg and mix well. Then add the vanilla and mix. Stop the machine and scrape down the sides and bottom of the bowl with a rubber spatula during these steps when needed to be sure they are all mixed.
Add the flour mixture and mix until incorporated.
Take 25 gram pieces ( a little bit more than 1 tablespoon can take) of the dough and roll them into balls. Alternatively, you can divide the dough equally into 22 pieces and roll them into balls as this recipe makes 22 cookies. Press the balls with a flat-bottomed glass or a measuring cup to flatten them evenly. Place on the baking sheet. Then, one by one, cover each side of the dough with toasted sesame seeds. You can gently use your fingers to make sure that sesame seeds stick to the dough. After you cover each dough with sesame seeds bake for 16 minutes until the center of the cookies become firm.
Remove the cookies from the oven and let them stay on the baking sheet for 20 minutes. As they cool, they become crunchy.