Pre-heat the oven to 350°F.
Line a 12 cup standard size muffin pan with cupcake liners.
In a medium mixing bowl, whisk together flour, baking powder and salt; set aside.
In another large mixing bowl, using an electric hand mixer (or in the bowl of an electric stand mixer using a paddle attachment), blend together butter and granulated sugar, on low speed, until combined.
Then whip until pale and fluffy on high speed; about 3 minutes; scrape down bowl.
Blend in the egg; then blend in the egg yolks and vanilla extract; scrape down bowl.
Add ⅓ of the flour mixture and mix, on low speed, just until combined; mix in milk; add another ⅓ of the flour mixture and blend, followed by buttermilk and blend; then lastly the other ⅓ of the flour mixture and mix until just combined; scrape down bowl and fold with a rubber spatula to ensure ingredients are evenly combined.
Transfer about ½ of the dough to a separate mixing bowl and fold the espresso powder in with that portion until its evenly blended; it may not mix in fully which is fine as it will dissolve as it bakes.
Scoop about 2 tbsp of the plain vanilla batter into cupcake liners and spread out even just a little.
Then top with about 2 tbsp of the espresso batter.
Place in the pre-heated oven and bake until toothpick inserted into center comes out clean; about 18 to 22 minutes.
Remove from the oven and allow to cool in muffin pan for a few minutes; remove and place on a wire rack to cool completely.
