Combine butter and sugar until creamy. Add in vanilla and egg until well-incorporated and then slowly add flour, baking powder, and salt. Shape dough into a ball. Cut into fourths and wrap each section tightly with plastic wrap and store in the refrigerator.
Scoop 1 teaspoon of dough and then roll in between the palms of your hands until a ball, about the size of a walnut, has formed. Drop onto a cookie sheet. Press your thumb into the center of the dough ball to form a slight well. Repeat until all dough has been used.
Place sheet pans in the freezer for about 12 minutes to firm to prevent spreading during baking.
Bake for about 12-15 minutes until brown. Remove from the oven and allow to cool slighty.
Scoop 1/2 teaspoon of your favorite jam, preserves, or fruit curd to fill the center of the cookie.
Store in an airtight container.
Chocolate Ganache
Pour heavy cream into a heavy bottomed sauce pan and bring to a low simmer over medium heat.
Break chocolate into pieces into a medium glass bowl. Pour cream over chocolate. Add butter and whisk until smooth.
Chill ganache for about 15-20 minutes to firm.
For easily adding the ganache to the cooled thumbprint cookie, pour ganache into a ziptop bag and snip the corner for easy dispensing. Alternately, you can use a pastry bag or a pastry bottle to dispense.