Position a rack in the center of the oven and preheat the oven to 350°F. Spray a 9 x 5-inch loaf pan with cooking spray and line it with parchment paper, leaving a 2-inch overhang on the long sides. Spray the parchment, too.
In a medium bowl, whisk together the flour and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-low until smooth and creamy, about 1 minute. Add the sugar; increase the mixer to medium-high and beat until light, fluffy, and doubled in volume, about 5 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Reduce the mixer to low and add the bourbon, then add the eggs one at a time, adding the next egg only after the previous one has been fully incorporated, scraping down the bottom and sides of the bowl after each addition. With the mixer on low, gradually add the dry ingredients and beat until just combined. Scrape down the bottom and sides of the bowl once more, and beat on low for an additional 30 seconds. Pour the batter into the prepared pan and use an offset spatula to smooth the top. Set the loaf pan on a sheet pan and bake for 60 to 65 minutes, or until a skewer inserted into the center of the cake comes out with a few crumbs attached. Place on a wire rack and immediately use the skewer to poke holes all over the cake.
As the cake cools, make the syrup. Combine the sugar, butter, and whiskey together in a small, heavy-bottomed sauce pan over medium-low heat. Cook until the butter melts, stirring slowly. The syrup will be thick and sugary.
If using a decorative loaf pan, you'll need to use a pastry brush to brush half the syrup into the holes, turn the cake out onto a serving platter while still warm (per the baker's notes), then brush the remaining half the syrup over the top of the cake. If using a traditional 9 x 5-inch loaf pan, there’s no need to turn out the loaf. Use the syrup completely—it will seem like there’s too much for the loaf. You’ll need to wait for the syrup to absorb some before brushing the cake with the syrup again. Serve warm, or at room temperature. The bourbon butter cake can be stored at room temperature, wrapped tightly in plastic wrap, for up to 3 days.