Put the chilli and butter in a small pan and melt on a very gentle heat to ensure the chilli flavour infuses while you make the rest of the dish.
Whisk the garlic into the yogurt. Season well and add the lemon juice. Bring a large pan of water to a simmer. Add the vinegar, make a swirl with a large spoon, then crack each egg into the water. Poach for 3 mins until set, then drain on kitchen paper.
To serve, put half the yogurt in each bowl, add a poached egg to each, then drizzle with the melted chilli butter. Season, top with sumac and coriander leaves, and serve with toast.