In a bowl, whisk together flour, salt, baking soda and spices; set aside.

In a large mixing bowl or the bowl of a stand mixer, beat butter and shortening together until smooth.

Add sugar and beat 1 to 2 minutes or until fluffy.

Beat in molasses.

Scrape down the sides of the bowl, then add egg and vanilla and mix until smooth.

Add half of dry ingredients and mix on low speed until just incorporated, scraping down the sides of the bowl as needed.

Add remaining dry ingredients in two additions until no streaks of flour remain.

The dough should be thick and just slightly sticky.

Cover bowl with plastic wrap; chill for at least 1 hour or overnight.

Preheat oven to 350 degrees F.

Line a cookie sheet with parchment paper or a silicone baking mat.

Roll dough into 1 inch balls (or use a small cookie scoop which is the perfect size).

Roll into a smooth ball, then roll in granulated sugar.

Arrange on prepared baking sheet, leaving 2 inches of space between balls.

Gently press an indentation into each cookie using your thumb or the back of a rounded ¼ teaspoon.

Bake for 7 to 9 minutes or until puffed and bottoms are just barely starting to darken.

Re-press the puffed indentations while the cookies are still warm, then let cool for 10 minutes before transferring to a wire rack to cool completely.

To make filling, gently melt chocolate together with butter and corn syrup in a saucepan over the lowest heat setting possible, or in the microwave on 50% power in 30 second intervals.

When chocolate is melted and smooth, transfer to a piping bag fitted with a ½-inch-round tip, then pipe into thumbprints.

Let sit at room temperature for about 1 hour or refrigerate briefly until chocolate is set.