Brown the butter. Add butter to a light-colored saucepan over medium heat. Melt, then continue cooking, stirring, until the milk solids turn golden brown and smell nutty, 5–8 minutes. Remove from heat and pour into a heatproof bowl, scraping in the browned bits.
Let cool to just warm, not hot, about 20–30 minutes.
Whisk the dry ingredients. In a separate bowl, whisk flour, cornstarch, baking soda, baking powder, and salt. If using cinnamon, add it now.
Mix sugars and butter. In a large bowl, stir together the brown sugar and granulated sugar. Add the cooled brown butter and whisk or beat until glossy and combined.
Add the wet ingredients. Whisk in the maple syrup, vanilla, egg, and egg yolk until smooth and slightly thickened.
Combine wet and dry. Add the flour mixture in two additions.
Stir gently with a spatula just until no dry streaks remain. If using nuts, fold them in now. Do not overmix.
Chill the dough. Cover and refrigerate for at least 45 minutes, ideally 1–2 hours. Chilling helps the cookies hold shape and develop deeper flavor.
Preheat and prep. Heat the oven to 350°F (175°C).
Line two baking sheets with parchment.
Shape. Scoop dough into 1 1/2-tablespoon balls (a heaping #40 scoop). Place 2 inches apart on prepared sheets.
Bake. Bake one sheet at a time for 9–11 minutes, until the edges are golden and the centers look slightly underbaked. They’ll set as they cool.
Optional: Pan-bang for perfect circles. If cookies are uneven, gently scoot a round cutter around the edges right after baking to neaten them.
Cool. Let cookies rest on the sheet for 5 minutes, then move to a rack to cool completely.
Optional glaze. Whisk powdered sugar, maple syrup, and a pinch of salt until smooth and drizzleable. Drizzle over cooled cookies.
Let set 15 minutes.