Note: this dough requires chilling.
Line a cookie sheet with a silpat baking mat or parchment paper.
Cream butter and both sugars in a stand mixer fitted with the paddle attachment. Mix in vanilla, egg, and water (or coffee). Add cocoa powder and pudding mix (the powder) and mix until smooth.
Whisk together the flour, baking soda, and salt. Add dry ingredients to the wet ingredients and mix until just combined. Stir in the M&Ms and sprinkles.
Scoop 2 tablespoon sized balls of dough onto prepared cookie sheet. You don’t have to space them out, this is just for chilling. Press a few M&Ms and sprinkles on the top of each cookie ball. (This is not mandatory, it just makes them look prettier.) Cover with plastic wrap and chill the cookies for at least 1 hour.
Preheat oven to 350°F. Prepare a second cookie sheet and place cookies about 2” apart on them. Press each cookie dough ball down slightly with the palm of your hand. Bake for 10-14 minutes, until they just start to lose their glossy sheen. I like mine slightly underdone, so I cooked them for about 11-12 minutes. I also like to rotate my cookie sheets in my oven halfway through for even baking.
Cool cookies on cookie sheet for 5 minutes, then transfer to a rack to cool completely. Store in an airtight container for up to 5 days or freeze for up to 3 months.