Bring the water and salt to a boil in a 28 cm / 11-inch heavy-bottomed pot.
Combine the buckwheat flour and cornmeal in a bowl and whisk together so they're evenly mixed.
Reduce the heat so the water is at a gentle simmer, around 90 C / 195 F. Add the flour mixture in a slow, steady stream, whisking constantly to stop lumps forming.
Once the mixture starts to thicken, switch to a wooden spoon or tarello. Stir continuously, scraping the bottom and sides of the pot, for 45 to 50 minutes over low heat.
The polenta is done when it pulls away cleanly from the sides of the pot and a spoon can stand upright in it without falling.
Remove the pot from the heat. Add the cubed butter and cheese, stirring until fully melted and the polenta looks glossy with long cheese strands running through it.
Pour the polenta onto a wooden board or large platter, spreading it into a mound about 3 cm / 1-inch thick.
Let it rest for 5 minutes to firm up slightly, then slice or spoon into portions and serve warm.