1-First, cream the softened butter for 1 minute, then add 66 grams (1/3 cup) granulated sugar and 25 grams (2 tablespoons) brown sugar, beating for 2 more minutes until fluffy.

2-Next, mix in 198 grams (2/3 cup) sweetened condensed milk, 1 large egg yolk, and 2 teaspoons vanilla extract to build a rich base.

3-Gradually add the dry ingredients: 255 grams (2 cups) all-purpose flour, 1 tablespoon cornstarch, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon cream of tartar, and 1/4 teaspoon salt, combining everything into a smooth dough.

4-Shaping and Baking Steps: Scoop heaping tablespoons of dough, roll into balls, and flatten to about 1/2 to 3/4 inch thickness on the prepared sheets.Bake for 8 minutes or until the edges turn golden, then let them cool completely this step is crucial for the cookies to hold the filling.

5-For the pastry cream, heat 1 1/4 cups of milk until bubbles form around the edges. In another bowl, whisk 4 large egg yolks with 396 grams (1 14-ounce can) sweetened condensed milk and 3 1/2 tablespoons cornstarch. Slowly add the hot milk to the egg mixture while whisking, then cook over medium heat, stirring until thickened. Remove from heat, stir in 4 tablespoons unsalted butter and 1 tablespoon vanilla extract, and chill for at least 2 hours.

6-To make the chocolate glaze, gently heat 198 grams (2/3 cup) sweetened condensed milk, 113 grams (2/3 cup) chopped chocolate, and 1/4 cup heavy cream until melted and combined. Let it cool to room temperature, adding extra cream if it thickens too much.

7-Finally, pipe the chilled pastry cream onto the cooled cookies and top with the chocolate glaze for that Boston Cream Cookies recipe with smooth custard filling and chocolate topping delight.