In a bowl, whisk together flour,
cocoa, baking soda, and salt. In
another bowl set over (not in) a
saucepan of simmering water,
place chocolate, butter, and brown
sugar; stir frequently until almost
completely melted. Remove from
heat, and stir until completely
melted; let cool slightly.
Add egg to chocolate mixture.
With a mixer on low, beat until well
blended. Gradually stir in flour
mixture (dough will form a ball).
Divide dough in half; roll out each
half on a sheet of parchment paper
to a 1/4-inch thickness. Transfer each
half (still on paper) to a baking sheet;
freeze until firm, about 20 minutes.
Preheat oven to 350 degrees. Working
with one half at a time, flip dough
onto a work surface; peel off paper.
Using a 2-inch heart-shaped cookie
cutter, cut out cookies; place, 1/2 inch
apart, on two baking sheets. Bake
until firm and fragrant, about 8 to 10
minutes. Transfer cookies to a
rack to cool.
To roll out the dough without using flour (which would mar its color), cover your work surface with parchment paper. Once the dough is frozen, you can peel off the paper and cut the cookies directly on the work surface.