Preheat the oven to 350°F / 175°C. Line a muffin tin with 6 parchment liners.
To a medium sized mixing bowl, add the flour, baking powder, salt, sugar, lime zest, and butter. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and only tiny clumps remain.
Add in the vegetable oil, egg white, buttermilk, and vanilla. Beat just until combined.
Distribute the batter evenly between the six liners. Bake for 15-20 min., or until a toothpick inserted in the center of the cupcakes comes out clean.
Use a toothpick to poke many small holes into the top of each cupcake. Use a pastry brush to brush rum over each of the cupcakes. Set aside to cool.
Mint Rum Cream Cheese Frosting
With a hand mixer or a stand mixer fitted with the paddle attachment, beat together the cream cheese, butter, and salt until light and fluffy (~2 min.)
Add half of the powdered sugar to the frosting and beat until combined, then repeat with the remaining half.
Add in the rum and mint extract and beat for an additional minute.
Once the cupcakes have completely cooled to room temperature, transfer the frosting to a piping bag and frost each cupcake. Top with key lime slices and fresh mint if desired. Serve and enjoy!
Notes
Because the frosting contains cream cheese, it does need to be stored in the fridge. I recommend waiting to frost the cupcakes until right before you serve them!