Step 1

In a medium bowl, beat the butter and sugar together until light and fluffy (at least 1 minute). Mix in the egg yolk and vanilla until combined. Gently stir in the cocoa powder. Then add the flour, baking soda, and salt. Mix just until combined. Stir in 2 tablespoons of the chocolate chips.

Step 2

Chill cookie dough in the fridge for at least 10 minutes.

Step 3

While the dough chills, adjust the cooking rack to the middle placement, select the BAKE setting, and preheat the toaster oven to 350°F. Lightly oil a small cookie sheet or line it with a silicone baking mat.

Step 4

Roll chilled dough into heaping tablespoon-sized balls. Place at least 2 inches apart on the prepared pan. Press the remaining 1 tablespoon of chocolate chips into the top of the dough.

Step 5

Bake until the cookies are set but the centers are still puffed and soft, 5 to 7 minutes. The cookies will continue to bake as they cool.

Step 6

Allow cookies to cool on the pan for 5 minutes before transferring them to a wire rack to cool completely.