Rinse buckwheat groats in a fine mesh strainer, under cold water, until the water is clear. Gently shake the strainer to get as much water out as possible.
Heat a medium-sized pan over medium heat. Add rinsed and well-drained buckwheat to the pan. Stir with a rubber spatula. Add a tablespoon of butter, and mix with a spatula until the groats are coated. Toast for about 3 to 5 minutes, stirring often.
When buckwheat groats are golden brown, carefully measure out 2 cups of boiling water and add to the pan. It will sizzle so be extra careful and go slow when pouring, adding one cup at a time.
Bring it back to a boil, season with salt, lower the heat to low, close the lid, and cook for about 13 min. Turn off the heat and let it sit for 10 min untouched and without removing the lid.
Remove the lid and fluff with a fork. Per preference, you can add ½ to 1 tbsps of butter and lightly mix it in. You can start with smaller amounts and add as you like or leave the butter out completely.
Notes
Check if buckwheat is cooked through at around the 10-minute mark. Insert a spatula or fork, If you don't see any liquid on the bottom of the pan - it's good to go. If there is still liquid it might need a few more minutes. Also allowing the buckwheat to stay with a lid for 10 minutes after cooking also allows all the remaining liquid to be absorbed.
If the buckwheat groats are already toasted, you can skip toasting it in butter and just cover it with boiling water.
Once cooked, buckwheat will keep in the refrigerator in an airtight container for several days.