In the bowl of a stand mixer, combine the room temperature butter, powdered sugar, and salt.  Using the paddle attachment start to cream on low for 2 minutes, then gradually increase the speed to medium high and beat until light and fluffy.

Add the egg yolk and beat for another few seconds until the yolk has disappeared.

Add the flour, working in two batches, and mix on low very carefully until everything is combined.

Finish the dough by hand with a spatula so as to not over work the dough.  It should still be a bit crumbly.  Pour the dough into a bowl and set aside.

Now repeat the above steps to make the pink shortbread.  Once the butter and sugar is creamed, add a few drops of the gel food coloring until the desired shade is reached.  (Remember it will lighten a bit after you add the flour.)

Continue with the egg yolk and flour and mix until combined and the color is mostly uniform. Set aside along with the first batch of dough.

Lay out a large piece of plastic wrap.  Using a small ice cream scoop or a spoon, scoop golf-ball sized pieces of dough gently smushing together in your hands to form a little clump and place closely together onto the plastic wrap, alternating pink/white/pink/white.  (I did about 4 or 5 rows of 6 creating a rectangle.)

Once complete gently knead the dough together a few times, with just a couple turns, to create the marbling effect. Using the plastic wrap to lift/manuever the dough is helpful. Don't worry about creating the perfect look as this will happen when rolling the dough out.

Once combined into sort of a disc, wrap the dough tightly with the plastic wrap and chill in the refrigerator at least an hour or freeze up to a couple months. (Double wrap the dough if freezing.)

When you're ready to roll and cut the cookies, remove the dough from the fridge or freezer.  If frozen, place in the refrigerator overnight, then let soften a bit on the counter.  

When the dough isn't rock solid, roll half the disc out between two pieces of parchment paper and cut the cookies out using your heart cutter or shape of choice.  (The dough can be re-rolled 2 times till there's hardly any left.)

Repeat with the other half of the disc.  Place the marbled, cut out pieces of dough& on parchment paper in the refrigerator to chill for a few minutes before baking.

Be sure to check the bottom of the cookies as sometimes the marbling effect looks best on one side as opposed to the other.

When ready to bake, bake in a preheated 350F/ 180C degree oven for about 10-12 minutes until the edges are just starting to turn golden. (Larger cookies may need a few minutes more to bake.) Let cool on the cookie sheet, then move to a cooling rack & enjoy.