Start by preheating your oven to 350°F (175°C). Lightly spray the bottom of a 9x13 inch baking pan with non-stick spray.
In a medium bowl, combine the sugar cookie mix, softened butter, 2 tablespoons of flour, and 1 large egg. Mix until you have a crumbly dough, then firmly press it evenly into the prepared baking pan to form your base.
Wash, remove the stems, and roughly chop your fresh strawberries. Spread them out evenly over the cookie crust.
In a separate medium bowl, whisk together the 3 large eggs, 1 cup of granulated sugar, heavy whipping cream, 1/4 cup of flour, 1/2 teaspoon of salt, and vanilla extract until the mixture is smooth and well combined.
Carefully pour this creamy custard mixture over the layer of strawberries in the pan.
Bake for 50-60 minutes. The edges should be set, and the center will have a very slight jiggle when gently shaken.
Allow the pie bars to cool completely to room temperature before slicing and serving. This is key for the custard to set perfectly.