Preheat the oven to 350ºF (180ºC).
Butter a cookie pan, or line baking sheets with parchment paper. Reserve. See Notes below.
Coarsely chop 5 ounces semisweet chocolate in a medium bowl and melt it with 2 tablespoons unsalted butter. You can do it in the microwave at 5-10’ spurts and mix it every time, or over a bowl with hot water, careful that the bottom doesn’t touch the water.
Beat 1 egg with ⅓ cup sugar in a large bowl for 3 minutes, until thick, pale, and tripled in volume.
Add ½ teaspoon vanilla extract and ½ teaspoon instant coffee and mix until creamy and shiny.
Add the melted chocolate mixture.
Incorporate the sifted 4 tablespoons all-purpose flour, ⅛ teaspoon salt, and ⅛ teaspoon baking powder. Mix until all is incorporated and no streaks remain. But don't overmix at this point.
Cover the dough and chill for 30 minutes.
Use a cookie scoop or a tablespoon to drop rounds onto the prepared pan, spacing them 1 inch and a half or 2 so the cookies can spread during baking.
Bake for about 10 minutes.
Remove when they're slightly crackled, shiny, and still slightly soft in the center. Please see the Notes below about testing baking time.
Let cool for a few minutes on a wire rack and carefully remove them. If too soft, let cool for another minute. But don't let them cool completely, or it will be hard to remove them without breaking them.
Store in airtight containers or cookie jars.