Preheat the oven to 350°F (180°C/Gas Mark 4). Line the bottom of a 10½-inch (26 cm) cake pan with a round of parchment paper and grease and flour the sides of the pan.. Melt the chocolate in a double boiler. (Alternatively, microwave the chocolate in a microwave-safe bowl in 30-second increments, stirring after each, until melted.) Remove from the heat and let cool.. Beat the egg whites with the lemon juice until stiff peaks form, about 5 minutes.. Beat the butter and granulated sugar in a bowl until light and fluffy, 2 – 3 minutes. Mix in the egg yolks, walnuts, and cooled chocolate, and mix together thoroughly. Fold in the whipped egg whites.. Pour the batter into the prepared pan and bake for 1 hour. Cover the top of the cake toward the end of the cooking time, if necessary. Allow to cool and transfer the cake to a serving dish and sprinkle the top with powdered sugar..