Preheat your oven to 325°F (170°C).
In a stand mixer fitted with a paddle attachment, add the unsalted butter and granulated sugar. Beat them together at medium speed until the mixture is light and fluffy, about 5 minutes. Don’t forget to scrape the sides of the bowl occasionally for an even blend. The texture should be creamy and pale.
Add the large eggs, one at a time, to the butter-sugar mixture, beating well after each addition. Include the vanilla extract as well; the batter might look slightly broken, but don’t worry—it will come together beautifully.
Transfer about half of this butter mixture (around 402 grams) to a medium bowl. This is where we’ll create our two different batters.
In a separate small bowl, whisk together 1¼ cups of cake flour, ¼ teaspoon of baking powder, and ¼ teaspoon of kosher salt. Fold this flour mixture into the first half of the butter mixture alternately with the whole buttermilk, starting and ending with the flour mixture. Mix gently until just combined to avoid overmixing.
In another small bowl, whisk together the remaining 1 cup of cake flour, ¼ teaspoon of baking powder, and ¼ teaspoon of kosher salt. Now, stir together the Roasted Strawberry Purée and red liquid food coloring. Then fold this mixture into the second half of the butter mixture, also alternating with the flour mixture. Mix until just combined.
Prepare a 10-cup Bundt pan by spraying it with cooking spray. Use a pastry brush to spread any excess evenly inside the pan, then lightly flour it, tapping out any excess. This step is crucial to ensure the cake releases easily after baking.
Using large pastry bags, spoon the plain and strawberry batters into separate bags. Cut a ½-inch opening at the tips. Pipe the batters randomly in a ring at the bottom of the prepared pan, alternating between the two to create a checkerboard-like pattern. Continue this layering until all the batter is used.
With a butter knife, gently swirl the batters together using a figure-8 motion. Take care not to touch the bottom or sides of the pan to keep the marbling distinct. Firmly tap the pan on a kitchen towel-lined counter several times to help settle the batters and release any trapped air bubbles.
Bake the cake in your preheated oven for about 52 to 58 minutes, rotating the pan halfway through. If you notice it browning too quickly, cover it loosely with foil during the last 5 to 6 minutes. The cake is ready when a wooden pick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely. This cooling step is important for achieving the right texture.
Once cooled, transfer the cake to a serving plate and generously spoon the Buttermilk Glaze over the top. If desired, garnish with fresh strawberries for a delightful finish. You can refrigerate any leftovers in an airtight container for up to 5 days; just remember to let it come to room temperature before serving.
