Spray the slow cooker with cooking spray. In a bowl, mix the flour, 1/2 cup granulated sugar, 2 tablespoons cocoa, the baking powder and salt. Stir in the milk, olive oil and vanilla. Scrape the batter into the slow cooker and smooth the top. In a small bowl, combine the brown sugar, 1/4 cup cocoa powder and the espresso powder. Whisk in the boiling water. Pour the mixture over the batter.
Cover and cook on high until a toothpick inserted into the center comes out clean, about 1 hour. Turn off the slow cooker, remove the lid and let the cake cool, 30 minutes.
Meanwhile, in a small bowl, whisk together the remaining 1/3 cup granulated sugar and 3 tablespoons cocoa. Whisk in the heavy cream. Cover and refrigerate for 30 minutes. Using a handheld electric mixer, beat the mixture until soft peaks form.
Scoop the warm cake and its sauce into bowls. Dollop the cocoa whipped cream on top.