Heat the oven to 150 C / 300 F. Line a 13 x 18-inch / 33 x 46 cm sheet pan with parchment paper, leaving overhang on the long sides.
In a large mixing bowl, stir together the buckwheat groats, rolled oats, chopped almonds, pumpkin seeds, cinnamon, and salt.
In a small saucepan over low heat, warm the honey, coconut oil, and almond butter together, whisking until smooth, about 2 minutes. Remove from heat and stir in the vanilla extract.
Pour the warm honey mixture over the dry ingredients. Stir with a spatula until every piece is coated and no dry patches remain.
Tip the mixture onto the lined sheet pan. Press it down firmly and evenly with the back of the spatula into a compact slab about 1 cm / 0.4 inch thick.
Bake for 35 to 40 minutes, rotating the pan once halfway through, until the edges are deep golden brown and the center feels firm to the touch.
Slide the pan onto a wire rack and let it cool completely, at least 1 hour, without touching it. Breaking it while warm turns clusters into crumbs.
Once fully cool, lift the slab out using the parchment overhang and break it into clusters of whatever size you like.