01 - Whisk together flour, baking powder, and salt in a medium bowl until fully combined.
02 - Using an electric mixer, beat softened butter and granulated sugar in a large bowl until the mixture is light and fluffy.
03 - Add eggs one at a time, mixing well after each addition, then blend in vanilla extract.
04 - Alternately add the flour mixture and milk to the creamed butter mixture, mixing just until a cohesive dough forms.
05 - Divide dough into two equal portions, flatten each into a disk, wrap in plastic, and refrigerate for 45 to 60 minutes.
06 - In a separate bowl, blend chopped chocolate, cocoa powder, light brown sugar, ground cinnamon, and melted butter until evenly mixed.
07 - Set oven temperature to 350°F (180°C) and line two baking sheets with parchment paper.
08 - On a lightly floured surface, roll each disk of dough into a rectangle approximately 1/4 inch thick.
09 - Distribute half the chocolate filling evenly over each rectangle, leaving a 1/2-inch border around the edges.
10 - Starting from a long edge, roll each filled rectangle into a tight log. Slice each log into 12 uniform cookies.
11 - Place cookies cut side down onto prepared baking sheets. Brush tops with beaten egg and sprinkle with coarse sugar or sprinkles as desired.
12 - Bake for 18 to 20 minutes until the cookies are golden and set. Cool on the baking sheets for 5 minutes, then transfer to wire racks to cool completely.