Preheat the oven to 350°F (175°C) and line the muffin pan with paper cupcake liners.
In a medium bowl whisk together the all-purpose flour, baking powder, and salt, then set aside.
In a large bowl beat the softened butter and granulated sugar with a mixer until light and fluffy, about 2–3 minutes.
Add the eggs one at a time to the butter mixture, beating well after each addition, then mix in the vanilla extract.
Add the dry ingredients to the wet mixture in two additions, alternating with the milk, mixing on low just until combined.
Divide the batter evenly into three medium bowls.
Leave the first bowl plain for the light base color.
In the second bowl mix in the unsweetened cocoa powder and 1–2 teaspoons of milk to make a smooth medium-brown batter.
In the third bowl mix in the dark cocoa powder (or extra cocoa) and enough milk drops to create a slightly thicker dark-brown batter.
Transfer each colored batter into separate piping bags fitted with small round tips.
Pipe a thin layer of plain batter into the bottom of each cupcake liner, just enough to cover the base.
Pipe irregular medium-brown spots over the plain batter, leaving space between each spot.
Pipe dark-brown batter in rough outlines or crescent shapes partially around each medium-brown spot to mimic leopard rosettes.
Gently cover the pattern with a thin layer of plain batter, being careful not to overfill the cups (about 2/3 full total).
Repeat the layering of medium-brown spots, dark outlines, and a light cover layer once more, finishing with plain batter on top.
Tap the pan lightly on the counter to release any large air bubbles without disturbing the pattern too much.
Bake the cupcakes for 16–18 minutes or until a toothpick inserted in the center comes out clean.
Remove the pan from the oven and let the cupcakes cool in the pan for 5 minutes.
Transfer the cupcakes to a wire rack to cool completely for about 25 minutes before frosting.
For the frosting, beat the softened butter in a bowl until creamy, then gradually add powdered sugar and salt.
Mix in the vanilla extract and 1 tablespoon of cream or milk, adding more as needed until the frosting is smooth and spreadable.
Once the cupcakes are completely cool, frost the tops with a thin layer of vanilla buttercream.
If desired, use leftover colored batters (baked as cookies or discarded) or colored frosting to pipe small leopard spots on top for decoration.