In a microwave safe, medium size bowl, cream together butter and sugar. Add flour and continue to mix until combined. Press into an ungreased loaf pan (8x4 or 9x5 will work). Bake in the oven for 15 minutes, until lightly browned on the edges.
Make the Filling:
While the base is baking, in the same bowl, melt the butter. Using a fork cream together the butter and sugar. Add the egg and continue to stir until well combined. Finally, stir in the vanilla, and salt.
Remove the base from the oven. If using add ins, sprinkle them evenly over the base. Pour the filling evenly over the base and fillings and return the pan to the oven for 19-20 minutes, until filling is nearly set.
Remove to a wire rack and gently run a butter knife around the edges of the pan. Allow to cool completely in the pan before cutting into bars.
Enjoy at room temperature or cold.
Notes
LeftoversStore in an airtight container at room temperature for up to 3 days, refrigerate for up to 1 week or freeze for up to 3 months.