To begin making the Whole Wheat Espresso Chocolate Streusel Cake Recipe, first get all the ingredients ready for all the steps.

To make the Streusel topping, combine all the ingredients in a bowl. Using your fingers make a coarse crumble of the mixture. Refrigerate the crumble until we are ready to use it.

To make the center layer of the Streusel, combine the sugar and nuts in a bowl and keep it aside.

The next step is the make the cake. 

Preheat the oven at 180 C. Grease and line 9-inch round cake pan.

In a double boiler, melt the chocolate. Once the chocolate melts, add the espresso to the melted chocolate and stir well to combine until it looks creamy and smooth. Keep the melted chocolate sauce aside.

Sift in a bowl, flour, baking powder, baking soda and cocoa powder and keep aside.

Beat together sugar, butter and sour cream using a hand mixer until just creamy. Do not over beat it. Add one egg and continue to beat until the eggs are fluffy. Gradually beat in the vanilla essence and espresso chocolate mixture. 

Finally gradually fold in the flour mixture and beat until the cake mixture is just about combined. Don't overbeat at this stage.

Pour 1/2 of the cake batter in into the cake pan. Spread the nuts and sugar over this layer and finally top it with the remaining cake batter. Finally sprinkle the refrigerated streusel topping evenly on the top and press it slightly.

Place the cake in the preheated oven and bake for about 55 to 60 minutes, until a tester inserted in the center comes out clean. 

Note: After 30 minutes, if you find the top over browning, just cover with aluminium foil to avoid burning.

Once done, remove from the oven and allow the cake to cool. Once cooled, the Whole Wheat Espresso Chocolate Cake is ready to be served.