• Heat oven to 350°F .
• 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.