In a large bowl, cream butter and brown sugar until light and fluffy. Combine flour and baking soda; add to creamed mixture with coconut and cracker crumbs until crumbly. Press half into a greased 11x7-in. baking dish.
For filling, in a large saucepan, combine sugar and cornstarch. Stir in water until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir for 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat; gently stir butter, lemon juice and salt.
Pour over crust. Top with reserve crumb mixture. Bake at 325° for 20 minutes. Cool. Spoon into dessert dishes. Serve with whipped topping if desired. Store in the refrigerator.