Turtle Thumbprint Cookies

For the Cookies:

unsalted butter, at room temperature

granulated sugar

large egg, separated

milk

vanilla extract

all-purpose flour

cocoa powder

salt

pecans, finely chopped

For the Caramel Thumbprint:

unwrapped caramel squares

whipping cream

Fleur de sel, or other flaked sea salt, for sprinkling, optional

For the Chocolate Drizzle:

semi-sweet chocolate chips

coconut oil or shortening