In a large mixing bowl, combine all of the flours together.

Cut cold butter into the flour blend using your hands or a pastry cutter until the mixture is crumbly.

Sprinkle with water and gently bring the dough together just until it forms a rough ball.

Press the dough evenly into the bottom of a greased 8x8-inch baking pan and set aside.

In a medium saucepan, bring butter and water to a rolling boil.

Remove from heat and quickly stir in the almond extract and flour.

Return to low heat and stir vigorously with a wooden or silicone spoon until the mixture forms a smooth ball, about one minute.

Remove from heat again and let cool slightly.

Beat in the eggs one at a time, mixing well after each addition until the batter is smooth and spreadable.

Spread the egg layer evenly over the prepared base layer.

Bake at 350°F for 50–60 minutes, or until the top is puffed, golden brown, and crisp.

Allow the pastry to cool completely in the pan.

In a small bowl, whisk together powdered sugar, almond extract, and enough milk to create a thick glaze.

Drizzle glaze over the cooled pastry and top with slivered almonds, if desired.