Preheat oven to 300 degrees F. Line two baking sheets with parchment paper; set aside.
In a small bowl, whisk together the flour, baking powder and salt; set aside.
Using an electric mixer, beat the butter and sugar on medium speed until light and fluffy, about 3 minutes. Scrape the sides of the bowl and add the eggnog, egg yolk and vanilla extract; continue beating on medium speed until completely combined, about 2 minutes. Reduce the mixer speed to low, add the dry ingredients and beat until just combined. Using a rubber spatula, fold in the chocolate chunks.
Using a small or medium cookie scoop (1 or 2 tablespoons), drop dough onto the prepared baking sheets, spacing them 2 inches apart. Bake from 20 to 25 minutes (longer for larger cookies), or until the edges are set and the centers spring back when touched lightly. Remove from oven and, using a spatula, transfer the cookies to a wire rack to cool completely.