Preheat oven to 350°F (180°C). Line an 8-inch (1.5 L) baking pan with parchment paper; set aside.
In a bowl, whisk together hot water and cocoa powder until smooth. Set aside to cool.
In another bowl, whisk together flour, baking powder and salt; set aside.
In a large bowl, using an electric mixer, beat butter until smooth. Beat in sugar until fluffy. Beat in eggs, one a time, until combined. Beat in vanilla. Stir in flour mixture in three additions, alternating with 2 additions of milk until smooth and well combined. Remove 1/3 cup (75 mL) of the batter and add to the cocoa mixture until smooth.
Spread remaining vanilla batter evenly into prepared pan. Spoon chocolate batter over top and using a knife swirl the chocolate mixture around to create a marble effect.
Bake for about 30 minutes or until cake tester inserted in centre comes out clean. Let cake cool completely.
Slice cake in half horizontally and carefully lift top off. Spread jam over bottom layer and top with cake. Spread sides and top liberally with icing to coat. Sprinkle top with chocolate cookie crumbs and decorate as desired.
In a large bowl and using an electric mixer, beat butter until light in colour.
Slowly beat in 1 cup (250 mL) of the icing sugar. Beat in cream and vanilla. Beat in remaining icing sugar and cocoa powder until fluffy and smooth.