For the peanut butter simple syrup: Combine the sugar, peanut butter and 1 cup water in a saucepan and simmer over medium heat until the sugar is dissolved and the mixture has thickened, 5 to 10 minutes. Set aside to cool.
Pour the vodka, hazelnut liqueur and 1 ounce of the peanut butter simple syrup into a shaker filled with ice. Shake well until chilled and frothy. Pour into small up glasses and garnish with a marshmallow that's been injected/stuffed with the peanut butter. Then try not to eat peanut butter from the jar with your hands.