Preheat oven to 300 degrees F.

In a stand mixer, beat butter and sugar until light and fluffy.

Add eggs one at a time, mixing well after each addition.

Stir in vanilla extract.

In a separate bowl, whisk together flour, cocoa powder, baking powder, salt, and cinnamon.

Gradually add half of the dry ingredients to the wet ingredients, mixing on low speed.

Stir in milk.

Add the remaining dry ingredients and mix until just combined.

Turn dough out onto a lightly floured surface.

Divide the dough in half.

Shape each half into a log approximately 12 inches long and 2 inches wide.

Arrange the logs on a buttered and floured baking sheet, spacing them about 3 inches apart.

Bake in the preheated oven for 40 minutes, or until the logs are firm and browned.

Remove from oven and let cool for a few minutes.

Using a serrated knife, slice the logs diagonally into 1/2-inch thick biscotti.

Arrange the biscotti slices back on the baking sheet.

Bake for approximately 10 minutes on each side, or until golden brown and crisp.

Cool the biscotti completely on a wire rack.

Store in an airtight container at room temperature for up to 2 weeks.