Make sure the butter is at room temperature. Soft, but not liquid.
Make the strawberry puree. Add 2 small strawberries (leaves removed) and a splash of water (less than 1 tbsp) to a blender. I found a cup-size blender (like a magic bullet) works best here. Blend until smooth. If the blender process isn't going well, add another splash of water. The puree should be smooth.
Add the butter to a large bowl and use hand beaters to beat until fluffy. I highly suggest using hand beaters over a stand mixer for this process because it is a small batch recipe. I found my Kitchen Aid stand mixer didn't produce the same fluffy results as the hand beaters.
Add 1 tbsp of the strawberry puree to the butter mixture. If you find you want a deeper colour, add 1/2 tbsp more. Beat until fluffy and incorporated into the butter.
Add the sifted powdered sugar 1/4 cup at a time. Beat until thick.
If you are piping the buttercream, place it in the freezer for 10 minutes to stiffen up before piping. If you are icing the cupcakes with a knife, the icing is ready as soon as it has been mixed with the hand beaters.
Make sure the cupcakes are completely cooled before icing them. Once cooled, either use a piping bag to frost the cupcakes or use a knife and spread evenly on.
Cupcakes are best fresh. Store in a sealed container on the counter.