Preheat your oven to 350°F (175°C). Line a 9-inch round cake pan with foil and spray with nonstick cooking spray.
Beat the butter and sugar in a large mixing bowl until creamy.
Add the egg and vanilla and mix until combined.
Mix in the baking soda, salt, and cream of tartar.
Slowly add the flour, mixing until combined.
Fold in the M&Ms and sprinkles.
Press the batter into the prepared pan and smooth the surface.
Bake for 19 to 22 minutes or until lightly golden.
Let the cake cool completely in the pan on a wire rack.
For the frosting, beat the butter until smooth and add the powdered sugar.
Mix in the salt, vanilla, and heavy cream until smooth.
Frost the cooled cake and decorate with more sprinkles and M&Ms.
