Preheat the oven to 350°F/180°C/gas mark 4. Have ready two ungreased cookie sheets. In a small bowl, mix together the flour and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until light, about 1 minute. Add the granulated sugar and beat for 1 minute. Add the whole egg and egg yolk and beat for 1 minute. Add the vanilla and beat until combined. Reduce the speed to low, add the flour mixture, and beat until combined. Here comes the fun part. Prepare your cookie press by placing the plunger on the tube part and turn to lock. Lift the plunger until it`s all the way at the top. Then fill the tube with the dough from the bottom opening. Fit the disk of your choice into the bottom ring and screw on tightly. Now press the gun part (it will click each time you press the handle) until you see dough just pushing against the disk at the bottom. Now you`re ready to press out dough. Place the end of the press firmly and evenly against a cookie sheet and press once on the gun. Lift the press. If the dough was adequately pressed against the disk, you should have a nice cookie shape on the sheet. If dough came out but did not stick, or if the shape is wonky, pick up the dough and put it back into the bowl (I told you this would be easy). Press out enough cookies to cover the cookie sheets, spacing them about 1 1/2 in/4 cm apart. Lightly sprinkle with colored sugar (if using). I usually sprinkle some with colored sugar and leave some plain. Bake until the cookies are light brown on the bottoms, 13 to 15 minutes. Remove to wire racks to cool completely. Repeat the process with the remaining dough, allowing the cookie sheets to cool completely. If the dough starts to seem too squishy to make defined shapes, refrigerate it for a few minutes. If you like melt the chocolate chips in a small saucepan over extremely low heat until just barely melted. Watch carefully to prevent burning. Remove from the heat and whisk until smooth. Line a cookie sheet with waxed paper. With a butter knife, spread a bit of melted chocolate on the bottoms of some of the cookies and press the cookies, chocolate-side down, onto the waxed paper. For sandwich cookies, spread melted chocolate on the bottom of a cookie and press the bottom of another cookie of the same shape onto the chocolate. Or drizzle melted chocolate on top of the shapes. Be creative! Let the cookies sit for about 1 hour to let the chocolate set. Store in an airtight container, with waxed paper between the layers, at room temperature for up to 7 days.