In a small saucepan, combine 1/4 cup (57 g) water, brown sugar, 2 tablespoons butter, orange zest, and salt. Warm gently over medium heat until the butter is melted and the sugar has dissolved. Set aside to cool until warm to the touch.
In a small cup, whisk together the remaining 2 tablespoons of water, the yeast, and the granulated sugar. Let sit for 5 minutes.
In the bowl of an electric mixer fitted with the paddle attachment, combine both wet mixtures. Whisk in the buttermilk. Add the all-purpose flour, rye flour, baking soda, and seeds. Mix on low speed until the dough comes together in a shaggy ball. Exchange the paddle attachment for the dough hook, and knead the dough on medium-low speed until smooth and elastic, 5 to 7 minutes.
Turn the dough out onto a work surface and knead by hand for a few minutes, the dough will be heavier and quite a bit denser than a white bread dough. Spray the mixer bowl with nonstick cooking spray or oil it lightly, and place the dough in the bowl. Cover tightly and allow to rise in a warm spot until doubled in bulk, 1 to 1 1/2 hours.
Line a baking sheet with parchment paper.
Remove the dough from the bowl to the work surface and shape into an oval with tapered ends, about 12 inches in length. Cover with plastic wrap and let rise again for 1 to 1 1/2 hours. Meanwhile, preheat the oven to 375°F.
Brush the top of the loaf with melted butter. Bake until golden and hollow-sounding when tapped, about 40 minutes, or until an instant-read thermometer reaches 190°F. Let cool completely on a wire rack before slicing and serving with lots of sweet butter.