In large bowl, whisk together flour, yeast, sugar, baking powder and salt. Slowly, stir in warm milk and melted butter until combined. On high speed, beat with electric mixer until batter is smooth and consistency is like thick pancake batter. Stir in 1/4 cup (60 mL) warm water until combined. Cover with plastic wrap and let batter rest at room temperature 1 to 1 1/2 hr. until it becomes bubbly and doubles in volume.
Stir batter to deflate air pockets. Heat 1 tbsp (15 mL) oil in large, heavy-bottomed skillet set to medium heat. Spooning 1/4 cup (60 mL) batter, pour into skillet in a round like a pancake. Cook in batches of 4, allowing 2 to 3 min. on first side, until bubbles appear on surface and bottoms are golden brown. Flip crumpets over and cook 1 to 2 min. until they're lightly golden on second side and cooked through. Cook remaining batter, adding remaining oil as required. Keep crumpets warm.
Pulse hazelnuts in food processor until finely ground. Add honey, vanilla and cinnamon. Pulse until almost smooth, scraping down sides as needed. Serve crumpets with hazelnut honey.
If not serving right away, cool and store crumpets in airtight container for up to 2 days or freeze up to 1 month. To reheat, toast crumpets or wrap in foil and heat through in warm oven.