How to set main class in build?

Upon sbt run I have multiple choices of main class.

I would like to set a main class so I've writen in build.sbt:

mainClass := Some("aMainClass")

But sbt fails with:

build.sbt:1: error: not found: value aMainClass

I've also tried with project/Project.scala file :

import sbt._
  class ExecutableProject(info: ProjectInfo) extends DefaultProject(info)  {
  override def mainClass = Some("aMainClass")
}

error :

 project/Project.scala:3: not found: type aMainClass

How to set the main class in a build?