Whisk together whole milk, egg yolks, granulated sugar, cornstarch, salt, and vanilla bean paste until smooth. Heat remaining milk until boiling, then slowly combine with yolk mixture. Return to heat and stir until thickened, about 5-7 minutes. Remove from heat and incorporate unsalted butter. Refrigerate until chilled.

Combine brown sugar and unsalted butter in a saucepan over medium heat, stirring until melted and bubbling for about 2-3 minutes. Slowly whisk in heavy cream and simmer for another 5 minutes until thick. Remove from heat and let cool.

Blend Medjool dates and unsulphured molasses until smooth. Cream together unsalted butter and light brown sugar in a bowl until fluffy. Add egg yolks and blended date mixture, mixing well. In another bowl, whisk together flour, baking soda, and salt. Gradually mix dry with wet ingredients, then let dough rest for 10 minutes.

Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop dough, roll into balls, and flatten slightly. Bake for 10-11 minutes until edges are golden. Cool on the sheet for a few minutes before transferring to a wire rack.

Pipe chilled vanilla bean custard buttercream onto each cooled cookie. Drizzle toffee sauce over the top and serve.