Make pastry cream: In small saucepan, heat milk and cream until barely simmering.
Meanwhile, whisk together sugar, flour, nutmeg, cloves and salt. Add yolks and whisk to combine. Whisk a little warm milk mixture into egg mixture. Then, whisking constantly, gradually whisk in remaining milk mixture.
Transfer entire mixture back to pan and cook on medium, whisking constantly, until it starts to thicken (whisking constantly ensures mixture stays as smooth as possible and yolks don’t scramble). When mixture is pudding-like consistency, start whisking occasionally until mixture comes to a slow boil. Remove from heat and whisk in vanilla.
Strain through sieve into measuring cup and press piece of parchment paper directly against surface. Refrigerate pastry cream until chilled, at least 2 hours and up to 2 days.
Make cream puffs: Heat oven to 400°F. In medium saucepan, combine butter, granulated sugar, 3/4 cup water and salt. Bring mixture to a boil, stirring occasionally to melt butter. Remove from heat and stir in flour and cinnamon. Return mixture to medium heat and cook, stirring constantly, until it pulls away from sides of pan, about 1 minute.
Transfer hot mixture to large bowl. Using electric mixer on low speed, beat 1 minute to cool slightly. Beat in eggs 1 at a time (dough will be smooth and shiny).
Transfer to freezer-safe resealable bag; snip off corner to make 3/4-inch opening. Pipe 1-inch mounds onto parchment-lined baking sheets, spacing 1 inch apart.
Bake, rotating pans halfway through, until puffed and golden brown, 20 to 25 minutes. Let cool completely on baking sheets.
Once cool, transfer pastry cream to piping bag fitted with round pastry tip and, working from bottom of each puff, fill puffs with pastry cream.
Pipe melted dark chocolate onto silicone baking mats or parchment paper for antlers and let set. Use melted milk chocolate to decorate tops for reindeer heads and eyes. Add nonpareils for spots and candies for noses. Transfer buttercream to small piping bag fitted with small star tip and use to pipe ears.