Preheat oven to 350 degrees. Using a hand mixer or stand mixer, cream the butter and sugar together until light and fluffy, about three minutes. Meanwhile, crack the eggs into a small bowl and then add them one at a time to the butter mixture, making sure to fully mix them in each time.
Add the flour, salt, and baking powder and beat until just combined. The dough will be super, super sticky. Line a baking sheet with parchment paper and dump the dough on it. Using your hands (I sprayed mine with cooking spray to make it easier to handle), pat the dough into a long rectangle, about 12 x 5 inches. This size will give you large biscotti. You can also halve the dough and create two rectangles for smaller pieces.
Bake the rectangle for 25 to 30 minutes, until lightly browned. Cool for 10 minutes on a wire rack. Using a very sharp knife, cut the rectangle diagonally to form 1/2 inch wide pieces. I got the best results using my fancy pants chef's knife (don't judge, but I totally sold Cutco in high school) and pushing straight down rather than sawing, though you might have a serrated knife that does the trick.
Place each slice cut side down on the baking sheet and bake for another 10 to 15 minutes. The biscotti should be lightly toasted and firm. Set aside to cool--as it cools, it will crisp up. When the biscotti have cooled, go ahead and slather with the peppermint bark (recipe below).