Preheat the oven to 350°. Line a muffin pan with cupcake liners and set aside.
In a large bowl whisk together the vegetable oil, warm coffee, egg, and vanilla until combined.
In a medium bowl sift together the flour, sugar, unsweetened cocoa powder, baking soda, and salt. Add the dry ingredients to the wet ingredients and gently whisk together until fully combined.
Scoop the batter into the prepared cupcake pan and only fill the cupcake liners 2/3 of the way full. Bake for 18-22 minutes, or until the cupcakes spring bake when lightly pressed. Cool completely before frosting.
Buttercream
In the bowl of a mixer add the butter. Use the paddle attachment to beat the butter for 1-2 minutes until it is smooth and creamy.
Add the powdered sugar and mix on low until most of the sugar has been absorbed by the butter. You may need to scrape down the sides and bottom of the bowl a few times. Add the vanilla and salt and beat on medium-high for 2-3 minutes.
Add the milk 1 tablespoon at a time on medium-low. Scrape down the sides of the bowl and then beat on medium-high for 2 minutes, or until the frosting is looking quite white and fluffy.
Separate a small amount of frosting into two small bowls and color one light pink and the other black.
Assembly
Using a large round piping tip, or the end of a piping bag snipped with a wide opening, pipe white frosting in one large, round dollop of frosting onto the top of each cupcake. Roll the cupcakes in a bowl of white sprinkles to evenly cover.
For each cupcake, cut a mini marshmallow in half and press it onto the bottom half of the top of the cupcake. Right above the mini marshmallow use a #10 round piping tip to pipe a small pink nose. Above the nose use a #8 round piping tip to pipe two eyes with black frosting.
To make the ears, cut one large marshmallow on a diagonal. Press the two sticky cut sides of the marshmallow into the pink sanding sugar and then place on the top half of the top of the cupcake as ears.