This is definitely not your average crumb cake. The surprise chocolate filling takes it to a whole new level.
Place all the crumble ingredients in a mixer with the beater attachment. Mix until crumbs form. (This mixture will not be dough-like. It should be crumbly.)
Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius) and prepare the filling.
To make the filling, put the chocolate, sugar, cocoa, water, and rum extract into a saucepot and bring it to a boil over a medium flame. This mixture needs a “babysitter” during cooking; the cocoa has a tendency to stick to the bottom of the pan, so stir constantly making sure the sauce doesn’t burn. When everything has melted, you should have a creamy chocolate sauce. Remove the pot from the flame, add the margarine, and stir it well until the margarine has melted. Continue stirring as you add the eggs and egg yolks, one by one. Mix thoroughly.
Pour the filling over the crumbs in the pan and bake for 10 minutes. Remove the pan from the oven and lower the oven to 320 degrees Fahrenheit (160 degrees Celsius). Spread the remaining crumbs evenly over the filling and bake for 45 minutes, until the cake has set and passed the toothpick test.
Let the cake cool. When completely cooled, remove the cake from the pan and cut it into 1½- x 1½-inch squares (first spread a disposable plastic tablecloth on the countertop to contain the mess). Place in cupcake holders and serve.
Line a large pan with Gefen Parchment Paper. Place two-thirds of the crumb mixture on the bottom of the pan, reserving one-third to be used later. Press the crumbs down with palm of your hand.
This cake can be made in advance and frozen before cutting. If you prefer cutting to size before freezing, make sure to freeze with plastic between the layers.