In a large bowl, beat the butter and sugar with an electric mixer on medium speed.
Add the eggs, vanilla and salt and continue blending until thoroughly combined.
Slowly add the flour and continue mixing until combined.
Roll the dough on parchment paper or silicone baking mats about a half of an inch thick and place it on baking sheets and let chill in the refrigerator for 1-2 hours slightly covered. Chilling the dough is important to get a sharp and precise shapes. Note that the dough can be refrigerated for up to 48 hours.
While the dough is chilling, preheat the oven to 350°F/180°C degrees.
Remove dough from the refrigerator and cut into desired shapes. Re-roll the extra dough edges and cut them with cookie cutter until the dough is used. If the dough becomes very soft, put back to the refrigerator to chill. The dough should be cold when going in the oven.
Bake for 10-12 minutes or until the cookies are set.
Allow cookies to cool on the baking sheet for 5 minutes. Then transfer to a wire rack to cool completely before decorating them.
To make the glaze icing:
Start by whisking the sifted confectioners' sugar, milk and corn syrup in a small bowl. Change the ratios of milk and confectioners' sugar to adjust consistency, but bare in mind that the icing should be quite thick.
Squeeze a small amount of food coloring into the glaze icing and use a toothpick or a bamboo skewer to drag the color across the surface.
Dip each cookie into the glaze icing, allowing access to drip off.
Place the cookies on a plate or cooling rack and allow the icing to set for about one hour. Until then, do not cover the cookies. Once the icing is set, you can enjoy the cookies or offer them as a great homemade gift!
Notes
Correctly measure out the flour - either weigh it using an electronic kitchen scale or spoon and level it with a knife but don't pack the flour in the measuring cup.
Do not substitute a stick of butter with a butter spread as the latter contains more oil and water in it and won't give you the needed dough consistency that should be soft and thick.
Make sure that eggs are at room temperature so they mix better with the rest of the ingredients.
Chill the rolled dough in the refrigerator and do not try speeding up the process by chilling them in the freezer as it won't chill evenly.
You can refrigerate the dough for up to 48 hours.
If the chilled dough starts to crack when you start cutting it, let it sit in room temperature to soften up a bit.
Don’t over-bake the cookies. If they are golden brown, it means they were baked for too long. They are done when they are set and no longer shiny, so don’t wait for the edges to turn.
If you prefer crunchier cookies rather than soft, leave them for one more minute in the oven.
In my experience, gel food coloring works best to decorate marble cookies.
This icing is really interesting. Each time it differs based on the ratio of milk to confectioners' sugar. The cookies' decoration varies accordingly. Note that the consistency of the icing should be quite thick.
If the glaze icing is too thick, add one teaspoon of milk. If it's too thin, add one teaspoon of confectioners' sugar.
Allow the cookies to completely cool before decorating them, otherwise the icing will melt.
Storing Tips:You can store decorated marble cookies or plain cookies in a covered container at room temperature for up to 5 days. Alternatively, you can store them in an airtight food container in the fridge for up to 10 days. This is the time period when the cookies stay nice and soft.Freezing: You can freeze decorated or plain cookies in a freezer-friendly container for up to 3 months. Put parchment paper between the cookies' layers so they don't stick to the next touching layer. Thaw at room temperature or in the refrigerator.You can also freeze the cookie dough for up to 3 months by wrapping rolled-out dough with plastic wrap and then placing it in an airtight container or plastic bag. Thaw in the refrigerator for at least one hour before working with it.You can store the glaze icing for up to 2 days in the refrigerator in an airtight container. Bring to room temperature before using.