Preheat the oven to 350F/175° C. Line 2 baking sheets with parchment paper.
In a medium bowl, sift together flour, salt, and baking powder. Set aside.
In a medium bowl, use a mixer fitted with the paddle attachment, beat butter and sugar on medium speed for 1 minute until light and fluffy. Beat in egg and vanilla extract just until combined. Scrape down the sides and up the bottom as needed. Reduce to low speed. Add dry mixture and beat just until combined. Do not overmix. Dough will be soft enough to work with. If the dough is too sticky to handle, try to mix in a little more flour. Wrap with plastic wrap and refrigerate for 1-2 hours until it’s firm enough for rolling. In this step, if you work as quickly as possible, you do not need to chill the dough.
Slightly soften the dough after you take the dough out of the fridge if using. On a lightly floured surface (or between 2 lightly floured parchment papers), roll the dough to a 1/4-inch thickness. Cut out shapes using a cookie cutter and place them on prepared baking sheets. Re-roll the remaining scraps into a ball and roll back out to cut the remaining cookies.
Bake cookies for 10-11 minutes, or until the edges begin to turn brown. Baking time may vary depending on the thickness of the cookies. Remove from the oven and allow cookies to cool on the baking sheets for 10 minutes, then transfer to a wire rack to cool completely.
To make the icing, in a medium bowl combine the powdered sugar with the corn syrup and milk and whisk until they’re well combined. Start with 2 tablespoons of milk. If it’s too thick, add 1 more teaspoon of milk. To thin out the icing add 1 more tablespoon powdered sugar. The icing is ready to use if it’s smooth and not too runny.
The icing can hold together for a few seconds and slowly drip after running off the spoon. Add gel food coloring as desired, drop by drop. Dividing it into smaller portions if you want to mix a few different colors. Cover the icing with plastic wrap or a wet tissue to prevent it from drying.
Draw a heart shape outline in the middle of cookies using No.1 decorating tip then continue filling the inside. Let the cookies with icing on top chill until the icing has set, then they are perfect for gifting. Store cookies at room temperature in an airtight container for up to 5 days or in the fridge for a week.