Place the butter in a large 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. Then add the egg and egg yolk, mix well. Between these steps stop the machine and scrape down the sides and the bottom of the bowl with rubber spatula to be sure they are all mixed.
Add the flour and just mix until the dough comes together. Divide the dough in half and cover them with plastic film, flatten and let them rest for 15 minutes.
Preheat the oven to 350°F(177°C). Line baking sheet with parchment paper or silicone baking mat.
Roll out the doughs about 0.3 inch (8mm) thick, 11,5*6,5 (29cm*16,5 cm) long. Use a 2 inch (5cm) wide heart cookie cutter to cut out the cookies. (If you don't have a heart cookie cutter, you can make them as circles or cut the dough into squares too.)
Put the egg white and sunflower seeds in different cups. Lightly beat the egg white with a fork. Remove the bubbles after you beat. Using a fork, first dip the cookies' both sides into the egg whites and then cover both sides with sunflower seeds.
Place them on the baking sheet and bake for 20 minutes or until the color gets darker and the cookies totally baked. Let the cookies cool for 10 minutes on the baking sheet before transferring them to a wire rack to completely cool.