Step 1

In a medium bowl beat the butter and sugar with a hand mixer until light and fluffy (about 2 minutes). Beat in egg yolk and extract.

Step 2

Place mixer on lowest speed and add cocoa powder, flour, baking soda and salt. Mix just until combined. Stir in chocolate chips and any additional mix-ins (see flavor variations in Notes section below).

Step 3

Chill cookie dough for at least 10 minutes.

Step 4

While the dough chills, preheat your toaster oven to 350°F and lightly oil your cookie sheet or line with a small silicone baking mat.

Step 5

Roll chilled dough into 6 tablespoon-sized balls. Place on prepared cookie sheet at least 2 inches apart and lightly flatten.

Step 6

Gently press extra chocolate chips into the top each cookie if desired.

Step 7

Bake for 5 to 7 minutes until the cookies are set but the centers are still puffed and soft. Allow cookies to cool for 2-3 minutes before transferring to a cooling rack.