Preheat the oven to 350°F/180°C, and lightly spray a mini muffin pan with baking spray.: You will notice a warm, dry scent as the oven reaches temperature, and the precise heat helps cookies set at the edges while staying tender inside. Preheating is important for even rise and consistent browning. A common mistake is skipping preheating, which can cause uneven texture and wider spread. Make sure the oven rack is centered so heat circulates evenly around the pan.
In a large bowl, using a hand electric mixer, beat the butter with the sugars until light and creamy. 2 minutes.: As you beat, you should see the color lighten and the mixture take on a slightly fluffy texture, with tiny air pockets forming that will translate into a lighter cookie. The sound is a steady mixing hum, and the aroma becomes buttery and sweet. If you under-cream, the cookies may be dense; over-creaming can cause excessive spread, so stop when the mixture is visibly paler and smooth.
Add the egg and vanilla extract, and beat until well combined.: The batter will become silkier and glossy as the egg emulsifies the fats and sugars. You might hear a faster, smoother mixer sound as the mixture becomes more cohesive. Ensure the egg is at room temperature to avoid chilling the butter and causing lumps. If curdling occurs, scrape the bowl and mix gently until smooth.
Add flour, baking powder, and salt, and beat again at low speed until just combined.: Once the dry ingredients join, the dough should transform from creamy to slightly thicker, with a uniform color and no streaks of flour. Use low speed to prevent gluten development, which keeps the cookies tender. Overmixing is the main error here, leading to chewier, tougher results. Stop mixing when you no longer see dry patches.
Using a small cookie scoop, divide the cookie dough between cookie cups.: As you portion dough into the mini muffin tin, you will feel soft, pliable dough hold its shape briefly before settling. I like to press each mound lightly so it nests in the cup; the sound is soft and doughy, without stickiness. Inaccurate scooping will result in uneven bake times, so keep the portion sizes consistent for uniform results.
Bake for 12 minutes. Remove from the oven, and as the cookie cups are still warm – using a measuring teaspoon push down the center of the cookie cup a little (this is optional but allows you add more frosting).: During baking, you will smell a caramelized sugar note and see the edges take on a gentle golden hue. Removing at the right moment is crucial; the centers should look set but slightly soft. Pressing the center while warm creates a shallow well for frosting and avoids cracking. If you wait until they cool, the cookie may harden and resist shaping, so work quickly but carefully to avoid burns.
In a large bowl, using an electric hand mixer, beat the butter until creamy then gradually add the powdered sugar to avoid a sugar storm.: The butter will appear pale and smooth, emitting a faint dairy aroma. As you slowly add powdered sugar , the texture will thicken into billowy frosting. Adding the sugar too fast can create a cloud of dust, and overbeating can warm the frosting and make it runny. Keep the mixer speed low while incorporating dry sugar for the cleanest result.
Add the milk, vanilla and food coloring if using, and beat again. Adjust the consistency if needed by adding more powdered sugar or milk.: Small splashes of milk will loosen the frosting, and the vanilla brings a sweet, aromatic finishing note. When you add color, do it drop by drop to control the shade. The frosting should hold soft peaks if lifted with a spatula; if it becomes too loose, add more powdered sugar in small amounts. A typical problem is oversaturating with liquid, which makes piping messy.
Frost the cookie cups, then decorate with mini eggs.: Use a small spatula or piping bag to deposit frosting into the warm wells, feeling the creamy texture spread smoothly into the indentation. The final step is tactile and visual; press the Cadbury mini eggs gently into the frosting so they nestle slightly without cracking. If frosting is too warm, it may slump, and if too cold, it will be stiff to spread. Aim for a spreadable, creamy balance so decorations adhere well.