Preheat the oven to 180C/350F/Gas 4. Line an 9x5" loaf pan with parchment paper. Set aside.
Whisk together the flour, sugar, baking powder, baking soda, and salt.
Whisk together the eggs, vanilla extract, butter, and sour cream. Pour the wet ingredients into the dry ingredients, and mix until just combined. Do not over-mix. Pour half of the mixture into a separate bowl, and fold in the cocoa powder.
Spoon a dollop of each mixture alternately into the prepared pan, then using a toothpick, swirl the two mixtures together. Bake for 50 - 60 minutes, or until a toothpick inserted into the centre comes out clean.
Cover loosely with aluminium foil halfway through baking to prevent the bread from browning too much.
For the Glaze
Place the chocolate chips, butter, and syrup in a heat-proof bowl, and microwave until completely melted and smooth, stirring every 10 seconds.
Drizzle the glaze over the cooled bread, and leave to set.
Notes
This bread stays fresh covered at room temperature for up to 3 days, or in the fridge for up to 7 days. It also freezes well for up to 3 months. Thaw overnight in the fridge.