Preheat the oven to 325 degrees F. Coat a tube pan or 15-cup bundt cake pan in butter, then dust with all purpose flour, ensuring the pan is coated. Tap off any excess flour (or you can use nonstick baking spray).
Combine butter, shortening, and sugar in the bowl of a stand mixer fitted with the paddle attachment. Cream together on medium speed for 5-7 minutes, until light and fluffy. Use a rubber spatula to scrape down the sides of the bowl and begin adding eggs, one a time, ensuring that each egg is fully incorporated prior to adding the next.
Scrape down the sides of the bowl again, then add the vanilla extract, butter vanilla emulsion, and lemon extract. Continue to mix on medium speed for 1-2 minutes, until all ingredients are combined and smooth.
In a separate bowl, whisk together creme fraiche and heavy cream until combined and smooth, about 10-15 seconds. In another large bowl, whisk together cake flour, kosher salt, and baking powder.
Add half of the dry ingredients to the wet ingredients in the stand mixer, mix for about 10 seconds, then pour in the crème fraîche mixture. Continue to mix for 15 seconds, then add the remaining flour mixture. Mix on medium speed for 10-20 seconds, until smooth. Scrape down the sides of the bowl again, ensuring the batter is evenly mixed together, then mix once more on low speed for 10 seconds.
Pour cake batter into the prepared pan and tap the pan against the counter to remove any air bubbles. Bake for 1 hour and 30-40 minutes, until a wooden skewer comes out clean. Allow the cake to cool in the pan for 15 minutes, then remove onto a wire rack to cool completely.
Notes
Use room-temperature ingredients – Butter, eggs, and other dairy should be at room temperature to ensure a smooth, even batter.
Measure accurately – Use a kitchen scale for precision or the spoon-and-level method for dry ingredients to avoid a dense cake.
Cream the butter and sugar properly – Beat them together until light and fluffy to incorporate air, which helps create a tender, light texture.
Check for doneness – Use a toothpick or skewer inserted into the center; it should come out with a few moist crumbs, not wet batter.