Pulse almond paste in a food processor until broken into small bits, then add salt and ¼ cup sugar and pulse until finely ground, about 1 minute. Preheat oven to 350 degrees. Butter a nine inch square baking pan then line with foil, leaving a 2 inch over hang on 2 opposite sides, and butter foil. Beat together butter and remaining ½ cup sugar in a large bowl with an electric mixer at medium speed until pale and fluffy, about 3 minutes. Add almond mixture, egg yolk, and almond extract and beat until well combined. Reduce speed to low and add flour. Mix. Spread batter evenly in the pan with a spatula. Lightly beat egg white in a small bowl, then brush some over the batter and sprinkle evenly with almonds. Bake until top is golden, 35 to 40 minutes. Cool completely in pan on a rack. Transfer to a cutting board with foil. Cut in squares.