Cream 3/4 cup vegan butter and 1/2 cup sugar in a mixing bowl for a minute until light and fluffy.
Make the vegan egg by mixing 1 1/2 tsp of Ener-G egg replacer powder with 1 tbsp water and stir until foamy. (This is replacing only one egg yolk, so I add only 1/2 of the water suggested on the Ener-G package).
Add the “egg” to the creamed butter and sugar and mix for another 30 seconds.
Add 1 1/2 cups flour ½ cup at a time while beating with a mixer until all of the flour is incorporated and you have a soft dough that is able to be rolled into balls without sticking to your hands.
Form small balls of dough by rolling in your hands and place on a cookie sheet.
Poke holes in the top of the balls. Use the back of a wooden spoon or the end of a rolling pin handle to make a hole that goes in almost to the bottom of the ball. (These holes will spread out and get bigger during the cooking process).
Bake at 350°F (177°C) for 12-14 minutes.
Let them cool on the tray for about 15 minutes.
Dust the cooled cookiesin a shallow bowl of powdered sugar.
Fill the hole with jam or jelly.
Notes
Don't actually use your thumbprint to poke the hole in or they will be too big and flat.
Cook until you start to see a few cracks in the top of the cookies, but they are not yet browned.
Once you have taken the cookies out of the oven and they are still warm, you can shape the divot with your finger to make it deeper or rounder.
Let them cool for at least 15 minutes before dusting them with powdered sugar so they don't crack and break.
Store in a sealed container on the countertop for up to 5 days or in the freezer for 3 months.
I find that the Ener-G egg replacer works the best in this recipe to replace the egg. It keeps the cookies white and pretty. You could also use a flax egg instead, which will bake up the same, but it will leave brown spots from the flax.