Preheat oven to 350F. Line muffin pans with 9 cupcake liners. Spray with cooking spray. Place one Oreo in the bottom of each liner.
Place butter and chocolate in a bowl. Set the bowl over a pan of boiling water. (Place 2 inches of water in a saucepan and bring to a boil. Set the bowl with the chocolate over the top. Make sure no water gets in the bowl or it will ruin the chocolate!) Stir until melted and smooth. Remove from heat and let cool slightly.
Add eggs, sugar, and pinch of salt to the bowl of a stand mixer. Beat about 5 minutes on level 3 until thick and yellow. (You can use a hand mixer...it will just take longer.)
While the mixer is running on low, slowly add the chocolate to the egg mixture. Mix until all the chocolate has been added, then turn the mixer off and fold the mixture until the egg has completely incorporated into the chocolate.
Scoop batter into prepared pan. I used a 1/4 cup ice cream scoop for this step. You can fill the cups almost to the top, these will not rise.
Bake for about 15-18 minutes, until it barely jiggles and a toothpick inserted in the center comes out with just a few crumbs. Cool before continuing.
While the mini pies are cooling, make the ganache by add the heavy cream and chocolate chips to a measuring cup or microwave safe bowl. Heat on HIGH for 30-40 seconds. Whisk until the chocolate is dissolved into the warm cream. Let sit at room temperature until it thickens, about 1-2 hours. (You can also speed up this process using the refrigerator.)
Spoon ganache over the top of the mini pies. Serve with a tall glass of milk!