In a medium-sized bowl, combine sourdough starter with water, sugar and salt. Add flour and stir well to combine.
Cover the bowl with a damp tea towel or plastic wrap and allow to ferment at room temperature for 8-12 hours. This can be done for 12-24 hours if placed in the fridge.
When ready to make crumpets, sprinkle baking powder over the batter and mix well to combine. The batter will start to become bubbly.
Heat a skillet or pan over low-medium heat. Grease the metal ring molds with butter, ghee or coconut oil. Add rings to the skillet to heat and add some more butter to the bottom of the pan.
Once hot, add crumpet batter using a small ladle or measuring cup, filling to 2/3 of the height of the ring. Do not over-fill the rings as the batter will overflow. Allow to cook over low-medium heat for 8-10 minutes. The batter will start to rise and create small holes/bubbles on the top.
Once the top of the crumpet is nice and bubbly, it will start to set on the top. The sides of the crumpet should have come away from the inside of the crumpet rings. Gently remove the metal rings with a pair of tongs.
Gently flip the crumpets using a spatula once the bottom of the crumpet is golden brown. Lightly cook the crumpets on the other side until golden brown, then remove from the pan and place on a cooling rack.
Repeat these steps with the remaining batter. I like to cool the metal rings in cold water before greasing with more butter before starting the next batch of crumpets.
You can eat these crumpets hot, straight from the pan if you wish. If you're not ready to eat the crumpets, store them in an airtight container or freeze in a zip-lock back for up to 3 months.
Reheat by placing in the toaster until heated through and crispy on both sides. Top with a knob of butter, some honey, or your favorite jam.