Prepare the Custard:**
- In a small saucepan, heat the milk over medium heat until it just begins to simmer.
- In a separate bowl, whisk together the sugar, egg yolks, and cornstarch until smooth and pale.
- Slowly pour the hot milk into the egg mixture, whisking constantly to prevent curdling.
- Return the mixture to the saucepan and cook over medium heat, stirring constantly, until the custard thickens, about 3-4 minutes.
- Remove from heat and stir in the vanilla extract and butter until smooth. Set aside to cool.
**Prepare the Cake:**
- Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan or line it with parchment paper.
- In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
- In a large mixing bowl, cream the butter and sugar together until light and fluffy, about 3 minutes.
- Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
- Gradually add the flour mixture to the butter mixture, alternating with the milk, beginning and ending with the flour. Mix until just combined.
- Gently fold in the blueberries.
**Assemble the Cake:**
- Pour half of the cake batter into the prepared cake pan and spread it evenly.
- Spoon the cooled custard over the batter, spreading it out evenly, but leaving a small border around the edges.
- Carefully spread the remaining cake batter over the custard layer, ensuring it covers the custard completely.
**Bake the Cake:**
- Bake for 40-45 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean.
- Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
**Serve:**
- Once cooled, dust the cake with powdered sugar if desired. Serve slices with a dollop of whipped cream or a scoop of vanilla ice cream.