Yields about 40 cookies.
2 cups flour
2 1/2 tsp ground cinnamon
1 tsp baking powder
1/4 tsp salt
1 cup sugar
3 Tbsp sugar
6 Tbsp unsalted butter, room temperature
2 large eggs
1 large egg yolk
1 tsp vanilla
Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper (or waxed paper).
Mix flour with 1 1/2 tsp cinnamon, baking powder and salt. Set aside. Beat 1 cup sugar and butter in a large bowl until light and fluffy. Add 1 egg, beat well. Add egg yolk, beat well. Mix in vanilla, then dry ingredients until just blended.
Transfer dough to a floured work surface (dough will be sticky...that's okay). Divide in half. Shape into logs that are about 9 inches long and 1 1/2 inches in diameter. Place on baking sheets. Beat remaining egg in a small bowl. Brush logs with egg. Bake until golden brown and firm to the touch, about 50 minutes (careful not to overbake). Cool on baking sheets for 10 minutes.
Mix 3 Tbsp sugar and 1 tsp cinnamon in a small bowl. Cut logs into 1/2-inch slices. Place cut-side down on baking sheets. Sprinkle cinnamon-sugar mixture onto cookies. Bake until pale golden brown, about 20 minutes. Cool on wire racks.