Preheat oven to 325°F/160°C.
Generously grease the cake pan.
In a small bowl, mix the flour and baking powder thoroughly.
In a stand mixer beat heavy cream, sugar, and eggs for 3-5 minutes until the mixture is light and fluffy.
With the mixer on low, gradually add the flour mixture. Mix just until combined.
Reserve 1/3 cup of batter in a small bowl and pour the remaining batter into the prepared pan.
Add the chocolate spread to the reserved batter, and whisk until combined, set aside.
Pour batter into the prepared pan.
Dollop the chocolate batter over the plain batter in the pan and gently swirl both batters together to make a marbled effect. Be careful not to over swirl- you'll lose the contrasting colors.
Bake for 25 to 30 minutes (it might take longer if you are using a loaf pan) or until a toothpick comes out with a few moist crumbs.
Let the cake cool in the pan for 20 minutes. Then, remove and cool completely on a wire rack.