• Heat oven to 350.
• Stir together 1 Tablespoon canola oil and 1 Tablespoon all-purpose flour in a small bowl. Brush this mixture lightly and evenly all over the inside of your bundt pan (avoid letting it pool in the bottom of the pan). Sprinkle 1/2 cup coarsely chopped toasted pecans over the bottom of the cake pan.
• Stir together 1/2 cup buttermilk (or 1/4 cup milk and 1/4 cup sour cream) and 1/2 cup rum in a measuring cup or small bowl. Set aside.
• Whisk 2 cups all-purpose flour, 1/4 cup cornstarch, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon salt together in a medium bowl. Set aside.
• Cream together the softened butter, 1 cup granulated sugar, and 3/4 cup brown sugar in the stand mixer at medium-high speed until light and fluffy, scraping down the sides of the bowl as needed. Add 4 large eggs, one at a time, continuing to beat at medium-high speed, scraping down the sides of the bowl before each egg is added. Add 2 teaspoons vanilla extract and beat again until it's completely incorporated.
• Add 1/3 of the dry ingredients, turn the mixer on to low speed, and slowly increase speed to medium until all of the flour has been incorporated. Add half of the rum mixture, turn the mixer on to low speed, and slowly increase speed to medium until all of the liquid has been incorporated. Repeat this process with the remaining 2/3 of the flour mixture and half of the rum mixture, finishing with the final 1/3 of the dry ingredients.
• Pour the batter into the prepared bundt pan and bake for 55-70 minutes, until the top of the cake is golden brown and a toothpick inserted into the center of the cake comes out clean. Let the cake cool in the pan for a few minutes.

Rum Syrup

• While the cake is cooking, stir together 1/2 cup unsalted butter, 1/4 cup water, 1/2 cup rum, 1/2 cup granulated sugar, 1/2 cup brown sugar, and 1/4 teaspoon salt in a small saucepan. Bring to a boil over medium-high heat (keep a close eye on it, it can boil over quickly). Reduce heat to medium-low and simmer for 5-8 minutes. Remove from heat and stir 1/2 teaspoon vanilla extract.
• Poke holes all over the cake with a skewer while it's still warm, and pour the syrup evenly over the cake. Cover the cake completely with plastic wrap and let it sit in the pan overnight.
• When you're ready to serve the cake, place a platter or cutting board on top of the pan, then quickly and carefully turn the pan upside down so the board is on the bottom. If the cake doesn't release, try heating it gently in the oven at 250-300°F for about 10 minutes. Cut into slices to serve.