Get Ready: Heat oven to 350°F (177°C). Line baking sheets with parchment paper.
Mix Dry Stuff: In a medium bowl, whisk flour, baking powder, and salt.
Cream Butter & Sugar: In a large bowl, beat cool room temperature butter and 1 ¼ cups sugar with an electric mixer on medium-high until light and fluffy (2-3 minutes). Scrape bowl.
Add Wet Stuff: Add egg, egg yolk, and vanilla. Beat until mixed.
Combine: Slowly beat in flour mixture on low until just combined. Do not overmix.
Scoop & Roll: Put remaining ¼ cup sugar in a dish. Use a scoop to make dough balls. Roll each in sugar.
Place & Flatten: Put dough balls on baking sheets, 2 inches apart. Flatten slightly.
Bake: Bake 10-12 minutes, until edges are set and light brown. Centers should look slightly soft.
Cool: Cool on baking sheets 5 minutes. Move to a wire rack to cool completely.