Rinse rice and urad dal in a mesh strainer.

Place rice, urad dal, and fenugreek seeds in a bowl.

Add water to cover and soak for 2-4 hours.

Drain, reserving the soaking water.

Grind rice and urad dal in a blender, using reserved soaking water to reach cake batter consistency.

Mix in salt.

Cover and place in a warm spot until batter is risen and fermented, 6-8 hours.

Stir fermented batter gently.

Heat 1 teaspoon oil on a griddle over medium heat.

Wipe quickly with a wet cloth to diffuse heat.

Spread a large ladleful of batter quickly, thinly, and evenly over the griddle using the back of the ladle.

Increase heat to high.

Spread 2 teaspoons of oil on the surface of the dosa and cover with a lid.

Cook until top is spongy, 1-2 minutes.

Remove lid and cook until bottom is crispy, about 1 minute more.

Spread 1 teaspoon of butter on top of the dosa.

Transfer to a plate.

Wipe griddle with a wet cloth and repeat with remaining batter.