In the bowl of an electric mixer, cream the sugar and butter until light and fluffy.
Beat in the egg and vanilla until creamy.
Gradually add flour, salt and baking powder. Beat until combined.
Remove 2/3 of the dough. Wrap the dough in plastic wrap and refrigerate at least 40 minutes to chill until easy to handle and no longer sticky.
To the 1/3 dough remaining in the mixer bowl, add the green food coloring gel. Mix to completely distribute.
On a sheet of plastic wrap, shape the colored dough into a 12” log. Wrap in the plastic wrap and freeze for about 30 minutes or until firm.
When ready to assemble,
Roll out the white dough between 2 sheets of wax paper to form a rectangle 12” x 6”. Place the green log along one long edge of the rectangle. Use the wax paper underneath to help roll up the green log in the plain dough, pressing the plain dough against the log as it rolls. Pinch together and smooth out the seam to seal.
Wrap the dough log in the wax paper and freeze for at least 15 minutes. The firmer the dough becomes, the easier it is to slice without becoming misshaped.
When ready to bake:
Preheat oven to 350° F. Line baking sheets with parchment paper.
Slice the dough log into ¼” thick slices and place 2” apart on the prepared baking sheets.
Bake for 8-10 minutes until edges are golden brown. Cool for 5 minutes on the baking sheet then transfer to a wire rack to cool completely.
Store at room temperature in an airtight container or freezer weight ziplock bag for up to 2 weeks.
Wrap stacks of 2-3 cookies in plastic wrap. Stack the wrapped bundles in a column in a freezer weight zip lock bag. Gently press out excess air before sealing.