Preheat the oven to 375F. In a large bowl add the butter and sugar and beat with a hand mixer on high until light and fluffy.
Add the egg and vanilla and continue to mix until blended.
In a medium bowl add the flour and salt and mix. Gradually add the flour mixture to the butter mixture and continue to blend with the hand-mixer. Remove 2 cups of the batter into a separate bowl.
In a small bowl add the semi-sweet chocolate and microwave on low heat for 30 seconds at a time mixing each time until melted. Add the chocolate to the remaining cookie batter and mix until blended. Cover both bowls of cookie batter with plastic wrap and chill in the fridge for about 15 minutes.
Remove the vanilla batter and roll out to a rectangle that is 1/2" thick. Cut the dough into 4" X 5" rectangles and set aside. Roll the chocolate dough into a 1" diameter stick and then cut into 5" lengths. Place the chocolate stick along the long edge of the vanilla dough rectangle and roll so that the chocolate is in the centre of the white dough and press gently to seal.
Insert one of the cookie rolls into the cookie press and insert the flower-shaped disc. Seal and press the cookies using the cookie press onto a non-stick cookie sheet. Bake for 5-6 minutes or until cookies are slightly golden brown on the bottom. Remove and carefully transfer to a cooling rack.
Storage:Store in an airtight container on the counter for up to 5 days or freeze for up to 3 months.