In a small bowl, combine the milk with the pomelo juice and set aside for about 5 minutes (this makes the madeleines extra fluffy). Meanwhile combine the sugar, melted butter, vanilla and zest. Pour in the milk mixture, whisking until combined.
Sift in the dry ingredients, making sure to avoid overmixing at this point. The batter should be fairly smooth but a few small errant lumps are alright. Cover the bowl with plastic wrap, making the plastic touch the surface of the batter. Chill in the fridge for at least 30 minutes or overnight for best results.
Combine some softened/melted butter with a bit of flour to create a slightly runny paste. Using a pastry brush or clean hands, lightly brush the mixture onto the madeleine mold, coating all the nooks and crannies. Take your madeleine batter and fill each mold ¾ full (avoid flattening, spooning/piping works best). Once the mold is full, place the pan in the freezer for a good 7 minutes.
Meanwhile, place a baking sheet in the oven and preheat to 350°F (180°C). When your oven is hot and ready, place the madeleine pan over the baking sheet and bake for 20-25 minutes or until the sides appear golden and the sponge lightly springs up when touched. Tap the pan on the counter at a slight angle then leave to cool for 5 minutes.
Make pomelo glaze by combining all the ingredients in a small bowl, adding more juice if the glaze is too thick. Unmold the madeleines, dip in the pomelo glaze and add some sprinkles. Serve warm with a cup of tea or a mug of good coffee. Enjoy!