In a small mixing bowl, mix together flour and baking powder. Set aside.
Add butter and both sugars to a large mixing bowl. Using an electric hand mixer, cream the sugars and butter together until light and fluffy. Add egg and vanilla extract and continue mixing until completely combined.
Add half of the flour mixture to the butter mixture, mix with a rubber spatula being careful to not over mix. Add the remaining flour and mix. You'll notice a cookie dough will form.
Use a small cookie scoop (2 teaspoon size), scoop the dough and roll into a ball using your hands. Place on a silicon mat lined or parchment paper-lined cookie sheet, about 1 inch apart.
Use the end of a clean, rounded butter knife to make the heart-shaped “thumbprint” on each cookie. Add about 1/4 teaspoon of jam or marmalade to each cookie.
Bake for 10 to 11 minutes at 350°F. Place the cookies on a cooling rack.
Make the glaze
Whisk the glaze ingredients together in a small mixing bowl.
You can use a spoon or you can add glaze into a zippered plastic bag and cut out a very small hole in one of the corners of the bag.
Drizzle the glaze over the cookies to your liking.
Notes
If you want, add a couple drops of pink icing color to the glaze and mix before drizzling like I did.