See instructions for making Swiss Meringue Buttercream.
Divide buttercream in half. Stir peanut butter into half and place in a pastry bag with no tip. Stir vanilla into other half and place into another pastry bag without a tip.
Fit a larger pastry bag with a large round tip , then snip the peanut butter and vanilla pastry bag then put them both into the larger pastry bag with tip, this will create the two tone buttercream swirl.
Squeeze gently until both flavours come out evenly then pipe a swirl onto the cupcake - to look like a soft serve ice cream.
Chill cupcakes in fridge for at least 45 minutes prior to glaze. It is important for the buttercream to be set and well chilled prior to the glaze, I actually leave it overnight.