In a cocktail shaker combine vodka, lemon juice, and Simple Syrup. Add ice cubes; cover and shake until very cold. Strain liquid into a chilled glass. Top with stout. If desired, garnish with a lemon twist.
In a small saucepan combine sugar and water. Bring to boiling over medium heat, stirring until sugar is dissolved. (The syrup is done when it is completely clear and the surface is covered with bubbles.) Remove from heat; cool. Store, covered, in the refrigerator up to 2 weeks.