Preheat oven to 325 degrees. Prepare a 9x5x3 loaf pan with a cooking spray with added flour in it, or grease and flour the pan. Set aside.
In a medium bowl, whisk together flour and baking powder. Set aside.
In a large bowl using an electric mixer beat butter for 30 seconds until smooth.
Gradually add in sugar a little at a time on medium speed until light and fluffy, around 6 minutes.
Add vanilla and beat for another 30 seconds.
Add eggs one at a time while beating 30 seconds per egg. (Make sure to scrape bowl with a spatula often.)
With mixer on low speed gradually add in flour mixture until combined.
Place 1/3 (approx. 1 1/2 cups) of cake batter into a medium bowl. Add chocolate syrup and mix well to combine. Add more for desired chocolate color.
Place 1/2 of the light colored batter into the prepared loaf pan. Place the chocolate batter on top. Place the remaining light colored batter over the chocolate layer, smoothing each layer as you go.
With a knife cut through all layers with a zig zag pattern creating the swirls.
Bake in preheated oven for 55-65 minutes or until toothpick inserted in center comes out clean.
Allow to rest for 5 minutes, then pull cake out of pan and cool completely on a wire rack.
Cake makes 12 slices.
