Preheat the oven to 175°C (350°F.)
In a small saucepan, bring the dates to a simmer with the stout. Cook for a few minutes, until the dates are very soft, then remove from the heat and stir in the baking soda. (It will get foamy.) Pour into a bowl, squish in the banana and roughly mash with a potato masher or a fork.
In a large bowl, beat the butter and sugar together until sandy; add the eggs and vanilla and beat for a few minutes, until pale and light.
In a small bowl, stir together the flour, baking powder and salt. Add half the dry ingredients and stir (or beat on low) just until blended. Stir in the date mixture, and then the remaining dry ingredients, stirring just until combined.
Pour the batter into a well greased Bundt pan or divide between 12 greased (not paper-lined) muffin tins.
Bake for 30 (for muffins) to 45 minutes (Bundt), or until deep golden and springy to the touch. Let cool slightly on a wire rack, and invert onto a plate while still warm.