01 -
Mix salt, flour, and cocoa powder in a bowl until combined.
02 -
Beat butter, brown sugar, egg yolk, and vanilla until fluffy.
03 -
Add flour mixture and red food coloring to make a soft dough.
04 -
Roll dough, wrap in plastic, chill for 30 minutes.
05 -
Heat oven to 300°F. Line baking sheet with parchment paper.
06 -
Form tablespoon-sized balls, roll in sugar, place 2 inches apart.
07 -
Make thumbprint in each cookie. Bake 10 minutes.
08 -
Mix filling ingredients until smooth.
09 -
Remove cookies, deepen prints if needed, add filling.
10 -
Bake 12 more minutes until filling sets.
11 -
Cool on sheet briefly, then move to wire rack.