Line a 9-by-13-inch baking sheet (quarter baking sheet) with parchment paper or a silicone baking mat.
For the buns: Combine the raisins with the hot water and set aside so the raisins rehydrate and plump.
Add the milk to the bowl of a stand mixer. Whisk in the yeast to dissolve. Sift in the flour and baking powder. Add the sugar, salt, cinnamon, nutmeg, softened butter, eggs and yolk and orange zest. Stir to combine.
Using the dough hook, knead for 7 minutes on medium-low speed. Increase speed to medium and knead until the dough is translucent when a small bit is stretched out with your fingers, 4 to 5 minutes longer. Drain the raisins and add them to the bowl along with the apple and knead until the dough is soft and elastic and the raisins and apple are evenly dispersed, about 2 minutes longer.
Lightly butter a large glass bowl. Scrape the dough into the bowl and cover with either plastic wrap or a clean dish cloth. Let the dough rise for about 1 hour. It will become puffy, but it's ok if it doesn't double in size.
Remove the dough from the bowl onto a lightly floured surface. Divide the dough into 15 parts. Using floured hands, form each part into a ball. Due to the raisins and apple, the ball won't be completely smooth, but that's ok. Add each ball to the lined baking sheet, spacing them evenly apart. Cover the baking sheet with plastic wrap and allow the buns to rise a second time until they're touching and look bloated and puffy, about 30 minutes.
Preheat the oven to 375 degrees F.
For the paste: Whisk together the flour and water in a small bowl until there are no lump. Spoon the paste into a piping bag fitted with a plain small to medium tip. (The batter will be runny.) Pipe a cross on each bun.
Bake until the buns are golden brown, 20 to 30 minutes.
For the honey butter: While the buns are baking, stir the melted butter and honey together in a small bowl. Once the buns are golden brown, remove them from the oven and brush the tops with the honey butter.
Carefully lift the parchment paper or baking mat and transfer the buns to a wire rack to cool. Allow to cool completely (or enjoy warm, if you're like me and you can't wait!).