Preheat the oven to 170°C. Grease a Bundt cake tin (or silicon mould).
In a large bowl sieve together flour, cocoa, baking powder and salt.
Mix the milk and sour cream in a mug.
With an electric mixer set to medium-high speed, beat the butter and sugar until pale and fluffy.
Add the eggs, one at a time, then add vanilla.
Add the flour mixture in batches, alternating with the milk mixture (finishing with the flour). Beat until just combined, with the mixer set to low speed.
Transfer the batter to the tin, making sure the top is smooth.
Bake for about 55 minutes, until an inserted skewer comes out clean.
Transfer the tin to a wirerack and let it to cool completely, before carefully turning it out onto a plate.
Make the glaze: put the chocolate bits in a heatproof bowl. Place the bowl above a pan of simmering water (don’t let the base of the bowl touch the water). When the chocolate has melted, take the bowl off the stove and let it cool a bit. Spoon the chocolate glaze over the cake and scatter some sprinkles on top. Let the glaze set before serving.