Use a thick frosting or buttercream as mortar. It fills gaps and sets hard once chilled.
Buckwheat flour is gluten-free and can be crumbly. If your cake has broken apart, you can still "put it together": buckwheat cake
To "put together a piece" of buckwheat cake , you can focus on the classic European style or repairing a broken layer if yours has crumbled. Classic Assembly (Italian/German Style) Traditional buckwheat cakes—like the Italian Torta di Grano Saraceno —are often assembled as a sandwich with a tart filling. Use a thick frosting or buttercream as mortar
Apply a thin layer of frosting over the assembled pieces to "seal" the cracks before doing a final decorative layer. If your cake has broken apart, you can
Dust the top with a heavy layer of powdered sugar right before serving. Fixing a Broken Piece