Preheat oven to 350 degrees and add liners (or grease) your cupcake pan
Mix the flour and baking powder and set aside
In the mixer, beat together the butter and sugar until light and fluffy
Add the eggs, one at a time making sure to mix well after each addition
Add the vanilla extract and rum, mix well
Reduce the mixer speed to low and add 1/3 of the flour. Mix until just incorporated
Add half of the milk continue mixing until incorporated
Repeat with 1/3 flour and the remaining milk
Add the last of the flour and mix well
Fill with batter to ⅔ each cupcake liner
Bake in oven for 20 to 22 minutes or until a toothpick comes out clean
For the Filling:
Combine the chopped strawberries and rum in a saucepan.
On medium heat, simmer the mixture about 5 minutes until the fruit softens.
Remove from heat and strain the mixture. Set aside the liquid to cool.
Add the fruit to a blender or food processor and puree until smooth.
For the Frosting:
Put a saucepan with 1 to 2 inches of water on medium high and bring to a simmer
In a grease-free mixing bowl, add the egg whites, salt, and sugar and whisk lightly to combine.
Set the bowl over the saucepan with the simmering water and heat whisking continuously until it reaches 160 degrees or until it no longer feels grainy between your fingers.
Whisk mixture on medium-high speed in the mixer until stiff peaks and mixture is to room temperature. It should be thick and glossy.
Remove whisk attachment and replace with the paddle attachment.
Set the mixer to low and slowly add the butter one tablespoon at a time.
Keep beating until the buttercream thickens and reaches a smooth consistency. It may look curdled at some point but keep beating and it will come together.
Add vanilla, strawberry liquid, and rum. Mix to combine.
Notes
To color icing (optional): Add a few drops of gel color icing in the color of choice.