Preheat the oven to 350 degrees. Spray a doughnut pan with non-stick cooking spray.
In a medium bowl, whisk together the buttermilk, egg, brown sugar, sugar, butter and vanilla.
In a large bowl, whisk the flour, cocoa powder, baking soda and salt. Pour the wet ingredients into the dry and mix until just combined.
Pour the batter into the cavity of each doughnut filling each about 3/4 of the way full.
Bake for 10 minutes, then place pan on a wire rack to cool.
While the doughnuts are cooling, prepare the glaze by whisking together the powdered sugar and cocoa powder. Add in the milk one tablespoon at a time until it is combined. Carefully whisk in the corn syrup until the glaze is thick and shiny.
Place a baking sheet lined with foil under the rack holding the doughnuts. Dip the tops of each cooled doughnut into the glaze and place back on the prepared rack, allowing the excess to drip off onto the foil. Cover with Monster Mashup sprinkles. Repeat with remaining doughnuts.
Doughnuts are best served the day of, but can be kept in an airtight container for 2-3 days.