¼ cup unsalted butter (1/2 stick), cold & cut into 4 pieces
1 teaspoon vanilla extract
½ cup sprinkles (not nonpareils)
6 Tablespoons unsalted butter, softened to room temperature
1 Tablespoon ground cinnamon
1 cup powdered (confectioners’) sugar
3 Tablespoons heavy cream
1 teaspoon vanilla extract
Make The Dough: In a small saucepan, warm the milk over low heat until lukewarm (no need to use a thermometer, but to be precise: about 95 degrees F). Pour the warm milk into the bowl of a stand electric mixer fitted with the dough hook attachment and manually stir in the sugar and yeast. Cover with a towel and let stand until the yeast is foamy, about 5-10 minutes. Add the softened butter, eggs, and salt. Scrape down the sides and manually stir as needed. Chunks of butter will remain. Gradually add the flour and beat at medium speed until a soft dough forms. Increase speed to medium-high and beat until the dough is soft and supple, about 6 minutes longer. *If you do not have a stand-mixer with a hook attachment, knead the dough by hand in this step.
Transfer the dough to a lightly floured surface and knead it with your hands for about 2 minutes. Form the dough into a ball and transfer it to a lightly greased bowl. Cover the dough and let sit in a warm place until doubled in size, 1 to 2 hours.
Line the bottom of a 9x13 inch baking dish with parchment paper, leaving room on the sides. Turn the dough out onto a lightly floured work surface and, using a rolling in, roll into a 10x24 inch rectangle. I used a ruler for accuracy. Make sure the dough is smooth and evenly thick, even at the corners.
Make The Filling: In a food processor, combine the sugar, flour, milk powder, and salt. Process for 10 seconds. Add the pieces of butter and pulse for 10 more seconds. Add the vanilla over the top and pulse until the mixture is crumbly, about 10 more seconds. Toss in the sprinkles with a fork.
Spread dough with 6 Tablespoons of softened butter and top with cinnamon. Sprinkle 3/4 cup of the cake mix filling overtop. There will be some leftover. See below for suggested uses. Tightly roll up the dough to form a 24-inch-long log. Cut into 15 even rolls (about 1.5 inches in width each). Arrange them in the prepared baking pan, cut sides up. Cover the rolls and let them rise in a warm place until they are puffy, about 2 hours.
Preheat the oven to 400F degrees. Cover the rolls with aluminum foil and bake for about 25 minutes, until they are golden. Transfer the pan to a rack to cool for about 15 minutes.
Make The Glaze: In a small bowl, whisk the confectioner’s sugar, heavy cream, and vanilla until smooth. Add more sugar or cream, depending how thick you want the glaze. Pour glaze over the rolls and serve warm.
Note: It's very important that you measure precise.