Preheat oven to 325 degrees F and arrange two baking racks in the upper portion of the oven. Line one or two baking sheets (depending whether youd like one big biscotti or 2 smaller ones) with parchment paper and set aside.
In a medium bowl, whisk together flour, cinnamon, baking powder and salt. Set aside.
Also whisk together the cinnamon and sugar for the topping and set aside.
In the bowl of a stand mixer, fitted with a paddle attachment, cream the butter and sugar until light and fluffy, about 4 minutes.
Scrape down the down and beat in the egg followed by the egg yolk.
Beat in the vanilla extract.
Add the dry ingredients to the creamed butter all at once. With the mixer or just with a spatula, bring all of the ingredients together until a somewhat stiff dough is formed.
For a large biscotti (about 12 cookies), shape dough into one large loaf, about 7-inches wide and use just one baking sheet. For smaller loaves (about 24 cookies), divide the dough in two on the two baking sheets. Shape each half of dough into a 9-inch long and 1 1/2-inch wide log.
Brush with the beaten egg and sprinkle very generously with cinnamon sugar (about 2/3 of the cinnamon sugar mixture). Bake the two sheets on two different racks in the oven for 20 minutes. Rotate the cookie sheets for even baking and bake for 20-25 more minutes until golden and firm to the touch.
Remove from the oven but keep the oven on. Let biscotti cool until able to handle. Using a serrated knife, cut logs into 1/2-inch wide diagonal slices.
Place biscotti cut side down on baking sheet and sprinkle with more cinnamon sugar. Bake again until pale golden, about 10-15 minutes.
Store in an airtight container for up to one week.
Go to full version of