Preheat the oven to 325 degrees. Line a large baking sheet with parchment paper.

In a medium bowl, whisk the flour, baking powder, and salt together.

In a small bowl, stir together the vanilla and espresso powder until dissolved.

To the bowl of a stand mixer, add the sugar and eggs + egg yolk and beat at medium speed until light and pale yellow, about 2 minutes.

Add in the melted butter, then the vanilla-espresso mixture, and beat just until smooth.

With the mixer on low speed, gradually add the flour mixture and beat until a crumbly dough forms.

Using a wooden spoon, stir in the chocolate chips, just enough to distribute.

Lightly dust work surface with flour and knead the dough for about 30 seconds until it begins to hold together. Divide the dough in half, then shape into two logs about 10 inches long and 1 inch high. Carefully transfer to prepared baking sheet, placing logs about 4 inches apart.

Brush each log with the egg white wash, then bake for 45 minutes or until light brown and firm to the touch. Cool on the baking sheet for 30 minutes.

Transfer logs to a cutting board and using a serrated knife slice, into small cookies about 1/2 inch think. Place 1/4 inch apart cut side down on baking sheet.

Bake for 7 minutes, then flip cookies over and continue baking for 7 more minutes, or until lightly toasted. Transfer to a wire rack to cool completely.

Place the 12 ounces of chocolate chips or melting chocolate discs in a microwave safe bowl. Heat for 30 seconds at 50% power, then stir. Continue heating in 20 second increments, stirring until completely melted and smooth. Dip half of each cookie into melted chocolate or use an off set spatula to ice the tops. Add sprinkles and place on wax paper until the chocolate hardens.