"This was given to me by my very best friend; it's her mother's recipe, and a wonderful basic biscotti recipe. Very rarely do I make them the same way twice. You can add dried cherries, cranberries, walnuts, toasted almonds, chocolate, white chocolate or cinnamon chips, or nothing at all -- It's good enough to stand on its own. I also change the flavorings and use almond or vanilla extracts. These will keep for a couple of weeks (if you're lucky) in your cookie jar!"
Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with parchment.
Beat sugar and butter together in a large bowl with an electric mixer until light and fluffy; beat in eggs, egg yolk, and vanilla extract. Whisk flour and baking powder together in a bowl. Stir flour mixture into butter mixture to form a fairly sticky dough.
Divide dough in half; with wet hands, shape dough into two loaves, about 1-inch high at the center, on the prepared baking sheet.
Bake in the preheated oven until lightly browned, about 20 minutes; remove to cool completely on a wire rack, about 30 minutes.
Slice loaves into 1/2-inch slices with a serrated knife. Place cookie slices back onto lined baking sheet, cut sides up.
Increase oven temperature to 450 degrees F (230 degrees C).
Bake in the preheated oven until golden brown, turning once, 3 to 4 minutes per side.