In a stand mixer using the paddle attachment, cream butter, salt and sugar. Add the egg and vanilla. Add flour (cocoa powder for the chocolate cookie dough). Mix until just combined. Wrap up and chill completely. Roll dough into a 3mm thick sheet, prick lightly with a fork and chill. Cut into flip flops and superimpose by 2; brush water to seal both sides. Chill an hour prior baking. Cookie dough can be frozen for weeks.
Bake at 375ºF/190ºC for about 20 min.
For the laces, roll out some marzipan or chocolate modeling. Seal each lace with melted white chocolate. Add M&M’s buttons if desired.
Flip flop cookies can be stored 3 days at room temperature; wrapped. Enjoy!