Preheat the oven to 350°F (177°C). Line two cupcake pans with 18 paper liners. Then, whisk the flour, baking powder, and salt together in a small bowl. Set aside.

Using a stand mixer or hand mixer, cream the butter and sugar in a large mixing bowl until the mixture is light and fluffy, about 3-4 minutes. Add the eggs one at a time until thoroughly mixed. Then, mix in the sour cream, oil, and vanilla bean paste.

In a liquid measuring cup, whisk the milk and instant coffee together until the coffee is dissolved.

Pour half the flour mixture into the wet ingredients, followed by half of the coffee-milk mixture. Mix on low just until combined. Repeat with remaining flour mixture and coffee-milk mixture, stirring just until the batter comes together and there are no more dry spots. Don't over-mix! Then, using a #20 scoop, divide the batter evenly between 18 cupcake liners.

Bake the cupcakes for 15-18 minutes, or until a toothpick inserted into the center of a cupcake has a few moist crumbs on it. Let the cupcakes cool for 5 minutes in the pan, and then transfer them to a wire rack to cool completely.

Using a stand mixer or hand mixer with a whip attachment, beat the butter and half of the powdered sugar together in a large mixing bowl until a thick frosting forms. Add the rest of the powdered sugar and beat until combined.

Add the milk, vanilla bean paste, and a pinch of salt. Turn the mixer on high and whip the frosting until it's light and fluffy, about 5 minutes. Scoop about 1/3 of the vanilla frosting into a small bowl and set aside.

Then, add the instant coffee and cocoa powder to the remaining 2/3 of the frosting and mix until smooth. Taste and add more instant coffee if you want a stronger coffee flavor.

Fit a reusable or disposable piping bag with a large tip. Then, use a rubber spatula to fill 1/3 of the piping bag with vanilla buttercream. Use another rubber spatula to fill the remaining 2/3 of the bag with coffee buttercream. Gently "squish" the bag to fill in any air pockets. It doesn't have to be perfect!

Twist off the top of the bag and pipe the cooled cupcakes with the frosting, adding more buttercream to the piping bag as needed. Enjoy the cupcakes immediately or store for later.