Preheat oven to 350°F (175°C). Grease two 9-inch round cake pans thoroughly and line bottoms with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
2½ cups all-purpose flour, 2½ tsp baking powder, ½ tsp baking soda, ¼ tsp salt
In a large bowl using an electric mixer, beat the softened butter and granular sweetener on medium-high speed until light and fluffy (about 3-4 minutes).
1 cup unsalted butter, 1½ cups granular erythritol-monk fruit sweetener blend
Add eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl.
4 large eggs
Beat in the vanilla extract until well combined, about 30 seconds.
2 tsp pure vanilla extract
In a separate small bowl or measuring cup, whisk together the milk and vinegar/lemon juice. Let sit for 1 minute to slightly curdle.
1 cup unsweetened milk, 1 tsp white vinegar or lemon juice
With the mixer on low speed, add about one-third of the dry ingredients to the butter mixture, followed by half of the milk mixture. Repeat, ending with the last third of the dry ingredients. Mix only until just combined – do not overmix.
Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula to ensure even baking.
Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean with a few moist crumbs. Start checking at the 25-minute mark, as sugar-free cakes can bake more quickly than traditional ones.
Let the cake cool in the pans on a wire rack for 10-15 minutes before inverting them onto the rack to cool completely.