Combine first six ingredients in medium bowl.. Mix thoroughly with whisk and set aside.
In mixing bowl, beat butter and brown sugar until creamy. Add egg and vanilla and beat on high until fluffy (about 1 minute). Gradually stir flour mixture into butter mixture until butter is just incorporated.
Turn the dough out onto wax paper (I’ve used plastic wrap, too). Use paper to shape and roll dough into cylinder about 10” long and 1 1⁄2 - 2” in diameter. Wrap tightly in more waxed paper, folding or twisting ends to seal. Chill for at least 45 minutes or freeze (up to 3 months) until needed. I find it best to chill dough overnight.
Before baking, preheat oven to 375. Unwrap dough (if frozen, let thaw 5 minutes) on a cutting board. Cut into slices about 1⁄4” thick. Place slices 1” apart on parchment lined baking sheet. Bake 10-12 minutes. Let cool until cookies are firm enough to transfer to rack to cool completely.