1
Heat the oven and prep the pan. Heat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment, leaving an overhang to lift the loaf out later.
2
Mix the dry ingredients. Whisk the flour, baking soda and salt in a bowl. Stir in the 130 g of chopped chocolate - tossing it in the flour helps stop it sinking to the bottom.
3
Cream the butter and sugar. Beat the softened butter and sugar on high for about 3 minutes, until pale and creamy.
4
Add the wet ingredients. Beat in the eggs one at a time, then the sour cream, mashed banana and vanilla until just combined.
5
Bring it together. Add the dry mixture and mix on low just until combined. The batter is thick - do not over-mix, or the loaf turns tough and gummy.
6
Fill and top. Scrape the batter into the pan and level it. Scatter the reserved 40 g of chopped chocolate down the centre.
7
Bake. Bake for 60-65 minutes. Tent loosely with foil at about 30-35 minutes so the top does not over-brown. It is done when a toothpick in the centre (between the chocolate) comes out with a few moist crumbs, not wet batter - start checking at 55 minutes.
8
Cool before slicing. Cool in the pan on a rack for at least 1 hour before lifting the loaf out and slicing. It keeps cooking as it rests, so slicing too early makes the centre seem gummy.