1. Make the Vanilla Pastry Cream:

• In a medium saucepan, heat milk until steaming, not boiling
• In a separate bowl, whisk together egg yolks, sugar, cornstarch, and salt until pale and smooth
• Slowly pour half of the hot milk into the yolk mixture while whisking constantly
• Return the entire mixture to the saucepan and cook over medium heat, stirring until it thickens and begins to bubble (3 to 5 minutes)
• Remove from heat and stir in butter and vanilla
• Pour into a bowl, cover with plastic wrap touching the surface, and refrigerate until fully chilled (at least 1 hour)

2. Make the Cookie Base:

• Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper
• Cream butter and sugar together until fluffy (about 2 minutes)
• Beat in eggs one at a time, followed by vanilla extract
• In a separate bowl, whisk together flour, baking powder, and salt
• Slowly mix dry ingredients into the butter mixture until combined
• Scoop about 1½ tablespoons of dough per cookie and place on baking sheets, leaving space between each
• Lightly flatten each ball for even baking
• Bake for 10 to 12 minutes or until edges are golden and centers are just set
• Cool completely on wire racks

3. Make the Chocolate Ganache:

• Heat cream in a small saucepan until steaming
• Pour over chopped chocolate in a heat-safe bowl
• Let sit for 2 minutes, then stir until smooth and glossy
• Allow to cool slightly until thickened but still spreadable

4. Assemble the Cookies:

• Place a teaspoon of chilled pastry cream onto the flat side of one cookie
• Top with a second cookie to create a sandwich
• Spoon or spread ganache over the top cookie and let set