Mix the dry ingredients: In a bowl, whisk flour, cornstarch, baking soda, baking powder, and salt.
Make the dough: Cream butter and sugars. Beat in egg, yolk, and vanilla. Slowly add dry ingredients.
Shape cookies: Scoop into balls, indent centers with a spoon, and chill for 30 minutes.
Prepare the filling: Melt butter, brown sugar, and maple syrup in a saucepan. Add pecans. Stir in cream and spices. Cool slightly.
Fill and bake: Spoon filling into each chilled dough ball. Bake at 350°F for 10–12 minutes.
Cool and finish: Let cool on tray, then move to a rack. Sprinkle with sea salt and enjoy.