Preheat the oven to 180 C / Gas 4. Grease and line a 900g loaf tin with baking parchment.
In a large bowl, combine the butter and sugar and whisk until smooth with an electric whisk. Add the eggs, one at a time, whisking well after each addition. Sift in the flour then add the baking powder, milk and vanilla extract. Whisk until well blended and light.
Toss half of the chocolate chips in a little flour to prevent against them sinking to the bottom of the loaf. Then fold them into the cake mixture. Pour into the prepared loaf tin and smooth the top.
Bake in the oven for 80 to 90 minutes, until a skewer inserted into the centre comes out clean. Remove from the oven and with the cake still in its tin, pour the remainder of the chips onto the top of the cake. Wait 5 minutes and then smear the melted chocolate over. Lift the cake out of the tin using the baking parchment to help you. Allow the chocolate to harden (about 1 hour) then slice and serve.