1) Preheat the oven to 180C/Gas 4.
2) In a small bowl, stir together the flour, baking powder, salt and rosemary. Set aside.
3) Using an electric mixer and a large mixing bowl, beat the butter and sugar together until light and fluffy, about 4 minutes. Add the eggs, one at a time, incorporating each egg fully before adding the next. Add the vanilla and gently stir in the flour mixture until just combined. Stir in the currants.
4) Cut the dough in half and shape into two logs. They should be about 30cm long and 2 1/2cm thick. Transfer to a greased baking sheet.
5) Bake until just starting to brown at the edges, about 35 minutes. Let cool for a few minutes on the baking sheets. While the biscotti logs are still warm, cut each log into 1 1/3-cm-wide slices on the diagonal. Place the slices back on the baking sheet (using two baking sheets if necessary to fit all of the slices). Bake for 7 to 8 minutes. Remove the baking sheets from the oven. Turn the biscotti over and bake for an additional 7 to 8 minutes, until the biscotti are crisp.
6) Let the biscotti cool for a few minutes on the baking sheet(s). Serve when cooled to room temperature, or store in an airtight container for up to two weeks.