1. Preheat oven to 350°F (175°C). Line a 12-cup muffin pan with cupcake liners.
2. In a medium bowl, whisk together flour, baking powder, and salt; set aside.
3. In a large bowl, beat butter and granulated sugar on low speed until combined, then whip on high for about 3 minutes until pale and fluffy. Scrape sides.
4. Beat in egg, then egg yolks, followed by vanilla extract; scrape bowl.
5. Add one-third of the flour mixture and mix on low speed until just combined. Add milk and mix.
6. Add another one-third of flour mixture, mix; add buttermilk, mix again. Add the last one-third of flour mixture and mix until just combined. Avoid over-mixing.
7. Transfer about half the batter (~12 ounces or 340 g) to another bowl and fold in espresso powder (some clumps are okay).
8. Spoon approximately 2 tablespoons of the plain vanilla batter into each cupcake liner, spreading slightly.
9. Top with approximately 2 tablespoons of the espresso batter, layering gently.
10. Bake 18 to 22 minutes, or until a toothpick inserted into the center comes out clean.
11. Cool cupcakes in the pan for a few minutes, then transfer to a wire rack to cool completely.

Optional Soak:
12. Mix espresso powder, coffee liqueur (if using), sugar, and hot water until dissolved.
13. Poke holes in cupcake tops with a fork or skewer and brush or drizzle soak over cupcakes. Allow soak to absorb before frosting.

Frosting:
14. Beat mascarpone cheese and powdered sugar until combined.
15. Gradually add heavy cream, whipping from medium-low to high speed until medium stiff peaks form.
16. Add vanilla extract and continue whipping until very stiff peaks form.
17. Pipe or spread frosting on cooled cupcakes.
18. Dust tops with cocoa powder before serving.