Saffron Bun Cake ("Saffransbutterkaka")

Directions

1. In a mortar and pestle, grind the saffron with 1 tablespoon of the sugar for the dough (alternatively, use a rolling pin to gently crush the saffron).
2. (alternatively, use a rolling pin to gently crush the saffron). In a small saucepan, warm the milk slightly until it reaches 98ºF. This isn’t necessary for the instant yeast, but the slightly warmed milk will help to bloom the saffron (this also makes it easy to substitute in active dry yeast, which would need to be activated with warm milk anyway). If it gets too hot, let it cool down before moving on to step 3 so it doesn’t kill the yeast.
3. In the bowl of a stand mixer, stir to combine the ground saffron and sugar, remaining sugar, yeast, and warmed milk. Add the flour, eggs, butter, and salt.
4. Attach the bread hook and knead on medium speed until the dough has come away from the bottom of the bowl and looks smooth, 25 to 35 minutes, depending on the power of your mixer. Trust the process and don’t add more flour unless absolutely necessary; it will look very wet to start, but by the time it’s done kneading, it will have come together fully.
5. Before you remove the dough from the mixer, test you’ve kneaded long enough by doing the window pane stretch test. Grab a piece of dough between your hands and gently stretch it; if you can stretch it thin enough to where light peaks through, it’s done. If it rips easily, keep kneading.
6. Cover the bowl with plastic wrap and set aside in a warm spot to proof until doubled in size, about 1 hour (see tip).
7. When you’re about ready to assemble, make the filling. In a small bowl, combine the butter, sugar, vanilla, and salt and grease a 9-inch springform pan.
8. Roll the dough out into a 12x16-inch rectangle, then spread the butter filling evenly to the edges. Tightly roll up lengthwise, the same way you would when making cinnamon buns. Divide into 9 equal sized pieces, and assemble in the cake pan.
9. Cover the buns and set aside to proof again until puffed, 30 to 45 minutes. If the dough springs back immediately when poked, keep proofing.
10. Preheat the oven to 350ºF/180ºC with a rack in the lower third position. Bake until golden brown and a kitchen thermometer inserted into the center reads at least 190ºF/ºC, 30 to 35 minutes (cover with tin foil if browning too quickly. No need to brush with egg wash.
11. Let the cake cool in the pan slightly, then remove and transfer to a wire rack or plate. Brush the top with melted butter and sprinkle with sugar.