To make the sponge, in the bowl of a stand mixer, whisk together the rye flour, warm water, honey and yeast. Cover and let sit until doubled and bubbly, about 1 hour.

Meanwhile, in a large skillet over medium, combine the pepitas and sesame seeds and toast, stirring, until the sesame seeds are golden (some pumpkin seeds will pop), 5 to 8 minutes. Measure out ½ cup of the mixture and set aside.

Transfer the rest to a food processor and process until finely ground, about 1 minute. Add the butter and process until just melted and combined, about another 20 seconds.

When the sponge is ready, add the bread flour, water and seed-butter mixture. Mix with the dough hook on low until just combined, about 1 minute. Let sit for 5 minutes.

Add the salt, then mix on low until the dough forms a mass around the hook, but still adheres to the sides, about 5 minutes.

The dough should look and feel sticky but not wet. Cover the bowl and let the rise until tripled in size, about 1 hour.

Heat the oven to 450°F with a rack in the middle position. Line a baking sheet with kitchen parchment. Turn the dough out onto a well-floured surface, being careful not to deflate it.

Lightly flour the top of the dough and gently press it into a 10-by-6-inch rectangle. To create 15 equal portions of dough (about 2 ounces each), cut the rectangle into thirds lengthwise, then into fifths crosswise.

Gently fold each portion into a ball, creating a smooth, taught surface and pinching together any seams on the bottom.

Arrange the rolls evenly on the baking sheet. Brush the tops generously with the egg and sprinkle the reserved seed mixture over them, pressing gently to adhere. Top each with a small sprinkle of salt. Cover, and let sit for 30 to 35 minutes, or until they have nearly doubled in size.

Bake until deep golden brown, 20 to 25 minutes, rotating the pan once halfway through. Using tongs, immediately transfer the rolls to a wire rack and cool at least 30 minutes before serving.