1) Preheat the oven to 425°F. Lightly grease two baking sheets, or line them with parchment.
2) In a large bowl, beat together the butter, sugar, vanilla, baking powder, and salt.
3) Beat in the egg. The mixture will look slightly curdled; that's OK.
4) Add the flour and currants or raisins, stirring until well combined.
5) Drop the dough by heaping teaspoonfuls onto the baking sheets, leaving about 1 1/2" between them. A teaspoon cookie scoop works very well here. Using the flat bottom of a glass dipped in sugar, flatten the dough balls slightly, to a scant 1/2" thick.
6) Bake the cookies for 6 to 8 minutes, until they're barely set, and a light golden brown around the edges. Remove them from the oven, and cool them right on the pan. If you need a pan to bake the remainder of the cookies, give the cookies about 5 minutes to set before transferring them to a rack to cool completely.
Yield: about 4 1/2 dozen cookies.