Step 1: Make the Cookie Base

1. Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
3. In a separate bowl, beat butter and sugar until light and fluffy (about 2 minutes).
4. Add the egg and vanilla extract, mixing until combined.
5. Mix in sour cream to keep the cookies moist and tender.
6. Gradually add the dry ingredients, stirring until a soft dough forms.
7. Drop tablespoon-sized scoops onto the baking sheet, spacing them 2 inches apart.
8. Bake for 10-12 minutes, or until the edges are lightly golden.
9. Let cool completely before assembling.

Step 2: Make the Vanilla Cream Filling

1. In a small saucepan, whisk milk, cornstarch, and sugar over medium heat.
2. In a separate bowl, beat the egg yolks.
3. Slowly pour some of the warm milk mixture into the egg yolks, whisking constantly (this prevents scrambling).
4. Pour the egg mixture back into the saucepan and cook for 2-3 minutes, stirring constantly until thickened.
5. Remove from heat and stir in vanilla extract and butter.
6. Transfer to a bowl, cover with plastic wrap (press it directly onto the surface to prevent skin forming), and chill for 30 minutes.

Step 3: Make the Chocolate Ganache

1. Place chopped chocolate in a heatproof bowl.
2. In a small saucepan, heat heavy cream until it just starts to simmer.
3. Pour the hot cream over the chocolate and let sit for 1 minute.
4. Stir gently until smooth and glossy.
5. Stir in butter for extra shine.
6. Let cool slightly before assembling.

Step 4: Assemble the Cookies

1. Spoon about 1 teaspoon of vanilla cream filling onto each cooled cookie.
2. Spread gently, leaving a small border around the edges.
3. Spoon melted chocolate ganache over the top, letting it drip slightly down the sides.
4. Let the cookies set for 15 minutes before serving.