Preheat oven to 350 degrees.
In a large mixing bowl, cream butter and sugar until light and fluffy, about 3-4 minutes. Beat in egg and vanilla.
In separate bowl, sift together flour, baking powder, and salt. Add to butter mixture and beat on low until just combined.
Turn dough out onto sheet of plastic wrap. Using plastic warp, press dough into disk and refrigerate for two hours.
Dust work surface liberally with powdered sugar and roll dough to ΒΌ inch thick. (Dough will get very soft and will easily stick to surface.) Alternatively you can roll dough between sheets of parchment paper.
Cut dough and bake on parchment lined sheets at 350 degrees for 7-8 minutes. Do not let cookies brown.
Cool on baking sheet for 2-3 minutes and transfer to wire rack to cool completely before glazing.
To glaze, mix glaze ingredients and spoon over cookies. Glaze will be dry to the touch after a couple of minutes.
To make piping, combine chocolate and oil in microwave-safe dish. Microwave on high for one minute and stir. Microwave in 20 second intervals until chocolate has melted. Transfer to piping bag to pipe.