Buckwheat flour is gluten-free and can be crumbly. If your cake has broken apart, you can still "put it together":

💡 If your cake is too crumbly to slice, try serving it in wedges with a dollop of whipped cream or crème fraîche to hide any imperfections. chocolate buckwheat cake – smitten kitchen

Carefully place the top layer back on. If it feels fragile, use a flat baking sheet to slide it into place.