In a small saucepan, melt butter with brown sugar, cinnamon, cornstarch, and vanilla. Stir until thick and smooth. Remove from heat and let cool completely. Once cooled, the filling should be scoopable, not runny.

Cream butter, brown sugar, and granulated sugar until light and fluffy. Add egg, vanilla, and milk, mixing until smooth.

Mix flour, baking powder, and salt separately, then add to the wet mixture just until combined. The dough should be soft but not sticky.

Chill the dough for about 20 minutes. This step prevents spreading and makes assembly easier.

Flatten a portion of dough, add a spoonful of brown sugar filling to the center, and gently wrap the dough around it. Seal completely and shape into a thick disc.

Bake until edges are set and tops look slightly puffed but pale. Overbaking will dry out the cookie and filling.

Once cooled, drizzle with a simple vanilla glaze. Let set before serving.