Step 1

Stir together buttermilk, sugar, and yeast in a medium bowl. Let stand until frothy and bubbly on surface, about 10 minutes. Meanwhile, combine all-purpose flour, spelt flour, and salt in the bowl of a stand mixer fitted with the whisk attachment; beat on low speed until combined, about 20 seconds

Step 2

Make a well in center of flour mixture; pour in buttermilk mixture. Beat on medium speed until batter is smooth and no lumps remain, about 2 minutes. Stir together 2/3 cup warm water and baking soda in a small bowl. Reduce mixer speed to medium-low; gradually beat in warm water mixture until batter is smooth and slightly thicker than buttermilk, about 30 seconds. If needed, add up to 2 additional tablespoons warm water, 1/2 tablespoons at a time, to thin batter to proper consistency. Cover bowl loosely with plastic wrap, and let stand at room temperature until surface is very bubbly and batter has doubled in volume, about 45 minutes.

Step 3

Heat a well-seasoned 12-inch cast-iron skillet over low. Brush skillet with 2 teaspoons oil; coat insides of 4 (3 1/8-inch) crumpet rings with cooking spray, and place in skillet. Let rings preheat until hot, 2 to 3 minutes. Carefully spoon about 1/3 cup batter into each ring, filling each ring halfway. Cook, undisturbed, until surface is covered in large holes and batter looks dry and slightly set around edges, about 8 minutes. Using tongs, carefully lift ring molds to remove. Using a small spatula, gently flip crumpets. Cook until bottoms are golden brown, 1 to 2 minutes.

Step 4

Transfer crumpets to a wire rack. Repeat process 2 times using remaining oil and batter, re-greasing and preheating rings between batches. Serve crumpets hot with desired toppings.