Pour half the melted butter into a 4-5 qt slow cooker.
In a medium bowl, mix the flour, baking powder, ¼ teaspoon salt, ⅔ cup sugar, 3 ½ tablespoon cocoa, remaining melted butter and milk until well combined. Pour into the slow cooker.
Mix the remaining sugar, brown sugar, remaining cocoa powder and remaining salt. Spread over the batter; do not stir.
Pour the boiling water on top of the chocolate cobbler. Do not mix.
Cook on low for 3 hours. The finished cobbler will be cakey on top and fudgy on the bottom. Serve with ice cream.