Preheat oven to 350 degrees F and line several baking sheets.
With an electric mixer, cream butter, sugar, baking soda, and salt.
Add black sesame paste, egg, and yolk, and mix until homogenous.
Add flour and gently stir until just combined.
Scoop 2 tbsp balls of cookie dough and place on your prepared baking sheets.
Bake 8-10 minutes (I tend to err on the side of underbaked, since these are thinner cookies and sesame paste makes them crisp up more than regular sugar cookies).
Let cool completely and enjoy! Leftover cookies can be kept at room temperature in an airtight container.
Notes
Black sesame paste, like nut butters, make cookies more crispy. 2 tbsp will give a lighter flavor but softer cookie; 3 tbsp gives a stronger flavor but a crispier cookie, similar to peanut butter cookies.