For the dough: Place the warm milk in a small bowl. Sprinkle the yeast over the milk and set aside for 5 minutes.
In a stand mixer fitted with the paddle attachment, cream the butter and granulated sugar until light and fluffy. Beat in the oil and vanilla. Beat in the yolks one at a time, beating well between additions. Beat on high speed for 2 to 3 minutes, then lower the speed and mix in the flour and salt. Mix in the milk and yeast mixture.
Change to a dough hook attachment and mix until a soft dough forms. Turn the dough out onto a floured work surface and gently knead it by hand for 2 to 3 minutes. The dough should be soft and smooth. Place the dough in a buttered or oiled bowl. Cover with plastic wrap and chill at least 8 hours or overnight.
For the filling: Heat the oven to 350 degrees F. Place the hazelnuts on a rimmed baking sheet and bake, stirring often, until golden brown and fragrant, about 20 minutes. Let cool completely. Place in a food processor and pulse until they are very finely ground; don't over-grind them or you'll end up with an oily paste. I like to add 1 tablespoon confectioners' sugar while pulsing. Set aside.
Cut the butter into equal-sized pieces. Put the butter in a light-colored saucepan (it's important to see the bottom of the pan to gauge the color of the butter as it cooks) and place it over medium heat. Melt the butter, stirring constantly. The butter will start to bubble and foam up; keep stirring. As the foam subsides, the color of the butter will begin to darken. As soon as it looks like it's browning, quickly pour it into a heatproof cup or bowl to prevent additional browning.
In another heatproof bowl, combine the brown sugar and cinnamon. In a small saucepan, heat the heavy cream, brown butter and salt. Bring to a boil and immediately pour it over the sugar-cinnamon mixture and stir to dissolve the sugar. Let cool completely and set aside.
Place the beaten egg in a small cup or bowl.
Butter a 9-by-13-inch baking pan. Turn the cold proofed dough out onto a floured work surface. Roll out into an 18x20-inch rectangle that's 1 inch thick. Spread the filling over the dough, leaving a 1-inch border around the edges to help seal the buns. Brush some of the beaten egg over the border.
Sprinkle the dough with the ground hazelnuts and pat down so it adheres. Begin rolling the dough up from the edge closest to you, keeping the roll tight. When you reach the egg wash part, pinch the dough together to seal the log. Roll the log a few times to even it out.
Cut the log into 16 pieces. Reshape each bun so it's round (cutting tends to flatten one side). Arrange the buns in 4 rows of 4 in the prepared dish, pressing down on the rolls to fill the gaps. Brush the tops lightly with beaten egg. Cover the buns with plastic wrap and allow to proof again until doubled in size, at least 1 hour.
Heat the oven to 375 degrees F. Bake the buns for 12 minutes, then rotate the dish and bake until the buns are cooked through, another 6 to 8 minutes (a digital thermometer into the center of the buns will read 185 degrees F when they are be fully baked).
Remove the pans from the oven and let cool on a wire rack for 10 to 15 minutes. Unmold and then let the buns cool completely.
For the malted icing: In a small bowl, combine the confectioners' sugar, cocoa powder and malted milk powder. Whisk in the heavy cream and vanilla. Drizzle the icing on the buns in a fun, playful manner. The icing will dry in 10 to 20 minutes. Serve immediately.
The brown butter can be made far in advance and kept refrigerated for several weeks. I use it for ice creams, waffle batters and butter creams, so it's very handy to have on hand.