01 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt.
02 - In a large bowl, use an electric mixer to cream softened unsalted butter and granulated sugar until light and fluffy. Add eggs one at a time, beating after each addition, then mix in vanilla extract.
03 - Alternately add the dry mixture and milk to the creamed butter mixture, mixing just until a soft dough forms.
04 - Divide the dough into two portions. Flatten each into a disk, wrap in plastic, and refrigerate for 45 to 60 minutes.
05 - While the dough chills, mix chopped chocolate or mini chips, cocoa powder, light brown sugar, ground cinnamon, and melted butter until the filling is even.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - On a lightly floured surface, roll each dough disk into a rectangle about 1/4 inch thick.
08 - Spread half the chocolate filling evenly over each rectangle, leaving a 1/2-inch border around the edges.
09 - Starting from the long side, roll each rectangle into a log. Slice each log into 12 cookies.
10 - Arrange cookies cut side down on prepared baking sheets. Brush tops with beaten egg and sprinkle coarse sugar or sprinkles, if desired.
11 - Bake for 18 to 20 minutes, or until cookies are golden and set. Cool on baking sheets for 5 minutes, then transfer to wire racks to cool completely.