Preheat oven to 325°F with convection fan // 350°F without fan // 180°C // Gas mark 4.
In a medium mixing bowl, whisk together flour, baking powder, salt and cinnamon. Set aside.
In the bowl of a stand mixer with the paddle attachment, cream butter and sugars together for 3 - 4 minutes, until pale and fluffy.
Then add eggs, one at a time. Beat until thoroughly combined.
Scrape down bowl, then add vanilla extract and mix to combine.
Add dry ingredients and mix until combined, then turn dough out onto lightly floured surface and shape into a rectangle approximately 13" x 3 1/2" x 3/4" (32.5cm x 9cm x 2cm) *If your dough has gotten too warm/soft, put it in the fridge for 20 - 30 minutes before baking.
Bake for 20-25 minutes. Remove from oven and let cool for 15 minutes. Slice log into 1/2" slices and lay them on their sides. Sprinkle cinnamon sugar over the cut sides. *See note
Return biscotti to the oven and bake for an additional 15 - 20 minutes.
Remove from oven and cool on baking sheet for a few minutes before moving to cooling rack to cool completely.