In a medium bowl, toss dates with baking soda. Pour boiling water over dates and set aside at room temperature for 1 hour.
Preheat oven to 325 degrees. Generously butter bottom and sides of an 8-inch square baking pan.
In a medium bowl, whisk together flour, salt and cocoa powder.
In a bowl of an electric mixer on medium high speed, beat butter until creamy, about 1 minute. Add sugar and beat until mixture is light and fluffy, about 2 minutes. Add eggs, 1 at a time, beating well after each addition. Add vanilla extract and beat until thoroughly combined. Reduce speed to low, add flour mixture and mix until just combined.