For the cake:

1. Preheat oven to 350º. Butter or spray a non-stick 9x5 inch loaf pan.
2. In a large bowl with an electric mixer, or the bowl of a stand mixer, cream butter with baking powder. Add sugar gradually and then walnut paste and beat for 2 or 3 minutes.
3. Add eggs, one at a time, beating well after each addition. Remove from the mixer and add flour in three additions, mixing well with a spatula.
4. Pour into prepared pan, level batter and add chocolate on top. Swirl with a knife, bringing some white batter to the top. Add crumble over batter and bake for 45 to 55 minutes or until a tester inserted in center comes out clean.
5. Cool on wire rack and unmold. It keeps, well wrapped in plastic, for a few days.

Walnut paste:

1. In the bowl of a food processor fitted with the metal blade, put walnuts and powdered sugar.
2. Process until well ground and mixed.
3. Add egg white and continue processing until a crumbly paste forms.
4. Keeps well covered and refrigerated for a few weeks.

For the streusel:

1. In the bowl of a food processor fitted with the metal blade, mix flour and sugar.
2. Add butter and process until crumbly.
3. Do not let it form a ball.