For the cookies:
1. Preheat oven to 325 degrees F.
2. Combine the flour, cocoa, baking soda, and salt in a small bowl. Set aside.
3. Beat the butter, granulated sugar, brown sugar, and vanilla extract in an electric mixer or food processor until creamy. If you don't have those appliances, use a whisk and mixing bowl—and be prepared for a good workout!
4. Add the eggs, one at a time, to the mixer or mixing bowl, beating well after each addition.
5. Gradually beat in the flour mixture.
6. Stir in the chocolate chips.
7. Use a tablespoon to drop the cookies onto an ungreased cookie sheet.
8. Bake for 11 to 13 minutes, or until cookies are puffed and centers are set. Cool on baking sheets for 2 minutes; remove to wire racks or plates to cool completely. Don't stack them or they will stick together.
For the mousse:
1. Whip the cream, powdered sugar, and salt together in a bowl with a whisk or an electric mixer until stiff peaks form. Add vanilla extract and whisk again.
2. Melt the chocolate with the butter in a bowl in the microwave for 30 seconds. Check it; if the mixture is not fully melted, put it in for another 15 seconds, and then check and stir again. Repeat if it's still not fully melted. (Some microwaves are more powerful than others.)
3. Use a rubber spatula or a spoon held upside down to fold the chocolate into the whipped cream mixture until it is fully incorporated.
4. Chill the mousse in the fridge for at least 2 hours, until fluffy.