In a small saucepan under medium low heat, add and melt the butter while stirring with a heatproof spatula. Increase heat to medium to bring the butter to a low boil while stirring intermittently. The butter will start to foam and then gradually brown as the milk solids become toasted and it takes on a rich, nutty aroma. (This whole process from melting to browning the butter will take about 3-5 minutes.)
Remove the browned butter from the heat and transfer into a heatproof mixing bowl making sure get all the browned bits. Cool the browned butter to room temperature. For reference, you should end up with a little over 4 ounces (about 8 tablespoons) of browned butter.
Preheat oven to 350F. Line a lightly greased 8x8 square baking pan with parchment paper. Set aside.
Into the bowl with the cooled browned butter, add the vanilla bean paste, egg, brown and granulated sugars. Whisk to combine. Next, add in the flour, baking soda, and salt. Switch to a spatula and mix the dry ingredients into the wet batter until a cookie dough forms. Mix in the chocolate chips.
Line the bottom of the prepared baking pan with the graham crackers sheets. You may need to break a few of the graham crackers sheets to fully fill the base of the pan.
Add two-thirds of the cookie dough over the graham crackers. Use a spatula or your hands to flatten into an even layer. Next, add the marshmallow pieces over the cookie dough layer and then top and fill in any gaps between the marshmallows with as much of the kinder chocolate pieces as you like.
With the remaining cookie dough, scoop or break off small pieces. Use hands gently flatten cookie dough pieces and place over the marshmallows and chocolate layer. Make sure to not completely cover the marshmallows. You want some of the marshmallows popping through the top cookie layer.
Bake assembled cookie bar for 24-26 minutes or until the bar is golden brown with crisp edges.
Remove the baking pan from the oven. Optional step - add a few of the kinder crispy creamy chocolate bar chucks over the hot cookie bar pressing down slightly to help adhere the chunks to the cookie bar. Transfer the baking pan back into the turned off oven using the residual heat to slightly melt the kinder chunks, about 1 minute.
Let the cookie bar set in the pan at room temperature for 10-15 minutes. Use the parchment paper lining to help carefully remove the cookie bar from the baking pan. Cut the bar into even pieces and serve while still warm for the best gooey texture. Enjoy!