Stir flour, baking powder and salt together in a small bowl.
In the bowl of a stand mixer (or using a hand mixer), beat butter with sugar until fluffy; beat in egg yolk and vanilla. (Reserve egg white for later use.)
After your dough is made, remove 1/3 of the dough and refrigerate the remaining. Color the 1/3 of the dough a pink or red color. Add 1-2 drops of food coloring if you're using liquid. If you're using gel color, use a toothpick to add your color until you have the shade you like. Mix with fork until all color is incorporated.
Roll out your colored dough with a rolling pin between two sheets of waxed paper. Refrigerate the red dough for 30 minutes.
After the dough is refrigerated and firm, use a small heart cookie cutter and cut out hearts until there is no more red dough (Re-rolling in between cutting). Refrigerate heart cut-outs for 30 minutes.
While hearts are firming in the refrigerator, roll the plain dough into even-sized logs about 1/2-inch longer than the hearts. This will give you 1/4-inch on each side of the hearts for the plain dough to overlap the ends.
Remove hearts from refrigerator and place them on their sides. Add logs to the string of hearts, brushing a small amount of egg white onto hearts before adding the log. Go all the way around the hearts until they are covered. Roll the finished log of dough around until it looks smooth.
Brush the outside of the log with more egg white and roll into some festive sprinkles or sanding sugar of your choice. Refrigerate for 1-2 hours.
When log of cookie dough is firm, remove from refrigerator and slice with a sharp, non-serrated knife into 1/4-inch cookies. Place on ungreased cookie sheet or cookie sheet lined with parchment paper. Bake for 10 minutes in a 350-degree F. oven until cookies are firm but not browned. Cool on wire racks.
Cookies can be stored in a container with a tight-fitting lid for 2-3 weeks at room temperature or frozen for up to 2 months.
Cookie dough can be wrapped tightly with plastic wrap and stored in the refrigerator for up to 1 week. Dough can be frozen for up to 6 months.
NOTE: Refrigeration/firming time is not included in the prep time.