Preheat your oven to 350°F (175°C). Grease and flour a 12-cup mini muffin tin and set aside.
In a medium mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs, one at a time, followed by the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, and mix until just combined.
Divide the batter evenly among the prepared mini muffin cups, filling each about two-thirds full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a cake comes out clean.
Remove the mini cakes from the oven and let them cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely.
While the cakes are cooling, prepare the creme egg filling. Cut the chilled Cadbury creme eggs in half lengthwise and scoop out the fondant filling into a mixing bowl.
Add the softened butter, powdered sugar, and milk to the bowl and beat until smooth and creamy. Add yellow food coloring, if desired, to achieve the desired color.
Using a piping bag or spoon, fill the centers of the cooled mini cakes with the creme egg filling.
Prepare the chocolate ganache by heating the heavy cream in a small saucepan until it just begins to simmer. Remove from heat and pour over the chocolate chips in a heatproof bowl. Let it sit for 2-3 minutes, then stir until smooth and glossy.
Dip the top of each mini cake into the chocolate ganache, allowing any excess ganache to drip off. Place a mini creme egg on top of each cake for decoration.
Let the chocolate ganache set before serving the mini creme egg cakes.