Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
In a medium bowl whisk together the flour, baking powder, baking soda, and salt.
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 to the butter mixture one at a time, beating well after each addition, then mix in the vanilla extract.
Stir the sour cream into the milk until smooth.
Add one-third of the dry ingredients to the butter mixture and mix on low just until combined.
Add half of the milk–sour cream mixture and mix briefly, then repeat alternating dry and wet ingredients, ending with dry, mixing only until just incorporated.
Divide the batter evenly among the 12 liners, filling each about two-thirds full.
Bake the cupcakes for 16–18 minutes or until a toothpick inserted in the center comes out clean.
Transfer the pan to a wire rack and let the cupcakes cool in the pan for 5 minutes.
Remove the cupcakes from the pan and let them cool completely on the wire rack, about 30–40 minutes.
While the cupcakes cool, beat the softened butter for the frosting in a large bowl until creamy and pale, about 2–3 minutes.
Gradually add the powdered sugar, one cup at a time, mixing on low until incorporated, then increasing speed to medium until fluffy.
Add the vanilla, salt, and 2 tablespoons of cream or milk, then beat until smooth and spreadable, adding an extra tablespoon of liquid if needed.
Divide the frosting into 3–4 small bowls and tint each with different shades of green gel food coloring, making one tiny portion pink and one tiny portion yellow if using.
Fit piping bags with star, round, and leaf tips, then fill each with a different shade of green frosting.
If using, spoon a thin layer of chocolate cookie crumbs onto the top of each cooled cupcake and press gently to create a “soil” base.
Pipe tall mounds or columns of green frosting in the center of each cupcake to form cactus shapes, using upward motions and small pulls to create texture.
Use the star and round tips to pipe small arms or additional cactus segments around the main cactus body.
Gently press white sprinkles into the cactus frosting to resemble spines.
With the pink and yellow frosting, pipe tiny flowers on top or on the sides of the cacti.
Chill the decorated cupcakes in the refrigerator for 10–15 minutes to help the frosting set before serving.