Bring raspberries to a boil over medium-low heat in a small saucepan. Simmer for 10 minutes, stirring frequently to mash up the berries. Transfer to a fine-mesh sieve and press down on the pulp to strain out the juice (you should have between 1/3 and 1/2 cup). Set juice aside to cool.
Butter and flour a 12-cup Bundt pan. Whisk flour, baking power, baking soda, and salt in a medium bowl to blend. Combine buttermilk, lemon juice, and zest in a small bowl.
In the bowl of a standing electric mixer, beat butter until smooth. Slowly add sugar and beat until well-blended. Beat in eggs, one at a time, scraping down the sides several times to ensure even mixing. Add vanilla extract and mix well. Stir in dry ingredients in 4 additions, alternating with the buttermilk mixture in 3 additions, occasionally scraping down the sides of the bowl.
Transfer about 2 cups of the batter to a small bowl. Fold in the raspberry juice until evenly combined.
Scrape one third of the lemon batter into the Bundt pan, gently spreading the thick batter to make an even layer. Top with half of the raspberry batter, doing your best to make an even layer. Repeat with remaining batter, ending with the lemon on top. Don't worry about swirling the batters together--that happens magically during baking. Tap the filled Bundt pan gently on the counter a few times to release any large bubbles in the batter.
Bake in an oven preheated to 350° F until pale golden and cracked on top, about 55 to 60 minutes. A tester inserted into the cake should come out with just a few crumbs attached.
While the cake bakes, prepare the lemon syrup. Combine lemon juice and sugar in a small saucepan over low heat. Stir until the sugar dissolves. Set aside.
When the cake finishes baking, transfer it to a rack and cool for 20 minutes in the pan. Turn out the cake on a wire rack. Immediately place the rack over a rimmed baking sheet and brush the syrup over the cake's hot surface, allowing the syrup to absorb completely before adding more. Cool cake to room temperature.
Whisk together confectioners' sugar, lemon juice, and salt in a small bowl. Add more lemon juice or sugar as needed to achieve desired consistency. The glaze should be thick but pourable. Drizzle glaze over the cooled cake and serve.
To make a 6-inch layer cake: Before mixing ingredients, butter three 6-inch cake pans and line them with parchment paper. Butter the paper and flour the pans. When ready to fill the pans, divide half the lemon batter between them. Distribute all of the the raspberry batter between the three pans. Top with remaining lemon batter. Bake for 40 to 45 minutes, until pale golden and a cake tester comes out clean. Add lemon syrup to cakes once cool right before frosting.
To make an 8-inch layer cake: Before mixing ingredients, butter two 8-inch cake pans and line them with parchment paper. Butter the paper and flour the pans. When ready to fill the pans, divide one third of the the lemon batter between them. Distribute half of the raspberry batter between the pans. Add another layer of lemon batter and then add the remaining raspberry batter. Top with the rest of the lemon batter. Bake for 35 to 40 minutes, until pale golden and a cake tester comes out clean. Add lemon syrup to cakes once cool right before frosting.
