heat oven to 350 F and line a 9 by 13 inch pan with parchment leaving overhang then lightly grease

mix oats flour brown sugar baking soda and salt in a bowl

stir in melted butter until evenly moistened and crumbly

press about two thirds of the mixture firmly into the pan in an even layer

bake until lightly set and just starting to turn golden at the edges about 10 minutes.

while crust bakes combine brown sugar butter cream and flour in a saucepan over medium heat and cook stirring until smooth and bubbling about 3 to 5 minutes

remove from heat and cool 5 minutes then stir in vanilla

whisk egg in a small bowl then slowly whisk in a few spoonfuls of warm filling to temper and whisk back into the saucepan until smooth

pour filling over hot crust and spread evenly

sprinkle remaining oat mixture over the top and gently press

add pecans if using.

bake until topping is golden and center is set but still slightly soft 18 to 20 minutes

cool completely in pan at least 2 hours for clean slices then lift out using parchment and cut into bars.