Preheat oven to 325°F and line an 8×8-inch pan.
Cream butter and sugars until smooth, add flax egg and vanilla.
Mix in baking powder, salt, and flour.
Combine filling ingredients in separate bowl.
Press half dough in pan, spread filling, top with remaining dough.
Bake 30–40 minutes until golden.
Cool, whisk icing, drizzle on top.
Slice into 8 bars and serve.
