Cut the butter into smaller pieces and place them in a mixing bowl. Add caster sugar gradually at the side of the bowl while mixing, beating thoroughly after each addition. Switch to medium speed to beat sugar and butter until the mixture fluffy and pale. Scrape the bowl down to make sure everything is thoroughly combined. Beat in egg yolk and orange extract until smooth. Add in the flour and fold to combine. Wrap up the cookie dough and chill for 1 hour.
Whish together the flour, cocoa powder and salt in a bowl. Set aside. Cut the butter into smaller pieces and place them in another mixing bowl. Add caster sugar gradually at the side of the bowl while mixing, beating thoroughly after each addition. Switch to medium speed to beat sugar and butter until the mixture fluffy and pale. Scrape the bowl down to make sure everything is thoroughly combined. Beat in egg yolk and rum extract until smooth. Add in the cocoa flour mixture and fold to combine. Wrap up the chocolate cookie dough and chill for 1 hour.
Divide each dough into 4 logs, about 1 inch in diameter, and reserve about 10 grams of each dough to make the dots. Press one side of each log with your thumb to make a comma shape. Cover and chill 1 hour until firm.
Dust a work surface lightly with flour. Place one orange dough log with round side down on the work surface. Brush the side lightly with water and turn the chocolate dough log on top of the orange dough, creating the yin-yang pattern. Gently roll the log until round. Repeat with the remaining logs. Wrap and chill 1-2 hours until firm.
Preheat the oven to 175C/350F. Cut the cookie logs into 5mm thick slices. Arrange the cookies on a parchment paper lined baking pan, about an inch apart. Pinch a bit of reserved chocolate dough, and roll it into a small round ball. Gently press it into the orange part of the cookies. Do the same with the reserved orange dough and press them into the chocolate part of the cookies. Bake in the center of preheated oven for about 12 minutes. Cool the cookies on the rack.
