Turn the slow cooker onto High and lightly grease the insert.
In a saucepan, combine the dates, water, butter, and baking soda. Heat gently until the butter has melted. Remove from the heat and allow the mixture to cool completely. To speed up the process, place the saucepan in a sink or bowl of cold water.
In a large bowl, beat together the brown sugar, egg, and vanilla until fluffy.
Add the cooled date mixture, flour, and baking powder, then mix until just combined.
Pour the batter into the prepared slow cooker.
In a saucepan, combine the water, brown sugar, butter, and vanilla. Heat gently until the butter has melted and the sugar has dissolved, then carefully pour the sauce evenly over the batter. Do not stir.
Place a clean tea towel under the slow cooker lid before replacing it to help absorb excess condensation.
Cook on High for approximately 1½ hours, or until the pudding is cooked through when tested with a skewer. Serve warm with cream, custard, or vanilla ice cream.