Preheat the oven to 350°F (175°C). Grease a bundt pan with cooking spray and evenly dust the inside with 3 tablespoons granulated sugar; set aside.
In a medium bowl, whisk together the 3 cups all-purpose flour, 1 teaspoon baking powder, and 1/4 teaspoon salt; set aside.
In a large bowl, beat 3/4 cup (1½ sticks) butter at medium speed with an electric mixer until light and fluffy. Gradually add 2 cups granulated sugar and 2 teaspoons vanilla extract, beating until blended.
Add the 3 large eggs one at a time, beating well after each addition.
With the mixer on low, add the flour mixture alternately with 1 cup milk, beginning and ending with the flour mixture. Beat just until combined; do not overmix.
Spoon about two-thirds of the batter (about 4 cups) into the prepared bundt pan and smooth the top.
To the remaining batter in the bowl, stir in 3/4 cup chocolate syrup and 1/4 teaspoon baking soda until just blended. Spoon the chocolate batter over the batter already in the pan.
Bake for 1 hour to 1 hour 15 minutes (about 60–75 minutes), until the cake pulls away from the pan sides and a toothpick inserted into the center comes out clean.
Let the cake cool in the pan about 15 minutes, then gently invert onto a wire rack and cool completely.
For the ganache, place 8 ounces chopped chocolate in a heatproof bowl. Bring 3/4 cup heavy cream and 2 tablespoons granulated sugar just to a boil, then pour over the chocolate and let sit 1 minute. Whisk until smooth, then stir in 2 tablespoons butter until melted.
Drizzle the warm ganache over the cooled cake, or drizzle over individual slices as desired.