Finely grate 2 tsp peel from lemon. Then squeeze 1 tbsp juice into a small bowl. Set aside. In a medium bowl, using a fork, stir flour with baking powder, salt and peel. In a large bowl, using an electric mixer, beat butter with sugar. Beat in egg, vanilla and juice. Using a wooden spoon, gradually beat in flour mixture just until it can be formed into a ball.
Knead in 3 oz (84 g) finely chopped dark chocolate. Divide dough into 3 portions. Shape each into an 8-in. log. Place 3 tbsp chopped pecans in a pie plate. Roll logs in nuts, gently pressing down so nuts stick. Wrap separately with plastic wrap and refrigerate until firm, at least 1 hour or up to 1 week.
To bake, preheat oven to 350F. Spray or oil a large baking sheet. Slice logs into rounds 1/3 in. thick. Place about 2 in. apart on baking sheet. Bake in centre of preheated oven until cookie edges are golden, 8 to 10 min. Remove cookies to a rack to cool completely. Repeat with remaining dough. Store in an airtight container in the refrigerator up to 1 week or freeze up to 1 month.