Preheat oven to 350°F. Line baking tray with parchment paper. Keep tray ready before mixing dough.

Add flour, baking powder and salt to a bowl. Mix gently and set aside.

Beat butter and sugar until light and fluffy. This helps cookies stay soft and tender.

Add egg and vanilla extract. Beat again until mixture looks smooth and creamy.

Add dry ingredients slowly. Pour milk and mix gently until soft cookie dough forms.

Scoop dough and place on tray. Leave space between cookies for spreading.

Bake for twelve minutes until edges set. Centers should stay soft and pale.

Remove cookies and cool on rack. Let them cool fully before filling.

Heat milk and sugar in saucepan. Stir gently until warm but not boiling.

Add cornstarch and egg yolk. Stir constantly until cream thickens smoothly.

Remove from heat. Add butter and vanilla. Mix until glossy and smooth.

Heat cream until warm. Pour over chocolate chips and stir until melted.

Spread cream on flat side of one cookie. Top with another cookie gently.

Spoon chocolate topping on top. Spread lightly without dripping sides.

Let cookies rest for ten minutes. Chocolate sets slightly and looks neat.