Authentic Danish Layer Cake has vanilla pudding and raspberry jam layered between soft yellow cake and is topped with lots of luscious buttercream frosting!
Preheat oven to 350 degrees. Grease and flour 2 (8-inch) round cake pans, then line parchment paper.
In a small bowl, whisk together milk, eggs, and vanilla. Set aside.
In the bowl of a standing mixer fit with the whisk attachment, beat together flour, sugar, baking powder, and salt.
With the mixer on medium-low speed, beat the butter in to the flour mixture one piece at a time. Continue beating until the mixture resembles moist crumbs, 1 to 3 minutes total.
Add the milk mixture to the mixer and beat until light and fluffy, 1 to 3 minutes longer. Stir by hand with a rubber spatula to make sure the batter is fully combined.
Divide the batter evenly between both pans, gently tapping the pans on the counter to settle the batter. Bake until a toothpick inserted in the center of the cake comes out dry with a few crumbs attached, 20 to 25 minutes. Rotate the pans halfway through baking time.
Cool the cakes in the pans for 10 minutes. Run a small sharp knife around the inside of each pan to loosen, then flip the cakes on to a wire rack. Peel off the parchment paper, then flip the cakes right side up to cool completely, about 2 hours.
In a small bowl, whisk together pudding mix and milk. Chill at least 10 minutes.
In a standing mixer fit with the paddle attachment, cream butter until pale and fluffy, about 3 minutes.
Reduce mixer speed to low. Add 4 cups powdered sugar, cream, vanilla, and salt (if using). Continue mixing 2 to 3 minutes longer. If the frosting seems too soft, add the remaining powdered sugar 2 tablespoons at a time.
Cover and refrigerate the frosting until assembling the cake, up to 1 week in advance.
Cut each cooled cake in half lengthwise so there are four layers of cake total.
Arrange one cake layer on a serving platter. Top with half the pudding and a second cake layer. Top with raspberry jam followed by a third cake layer. Spread the remaining pudding and top with a final cake layer.
Frost the cake with buttercream and garnish with fresh raspberries if desired. Store leftovers in the refrigerator.