In a small saucepan over low heat, bring the milk to a simmer. Remove from heat and immediately add the lavender. Set aside to steep for 20 minutes. Strain the milk through a fine-mesh sieve set over a bowl, then discard the lavender. Cool milk completely before using in cake batter. Can be made 1-2 days ahead and stored in the refrigerator. Bring to room temperature before using.

In a small saucepan over medium-high heat, bring the sugar and 1/2 cup water to a boil. Once boiling, reduce the heat to low and add the lavender. Simmer the syrup for 10 minutes. Remove from heat and steep until completely cool. Strain the syrup through a fine-mesh sieve set over a bowl, then discard the lavender. You will use this “soaking syrup” in step 8. Can be made 1-2 days ahead. Cover and keep at room temperature until ready to use.

Preheat the oven to 350°F (177°C). Grease three 8-inch cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

Stir the sour cream and cooled lavender milk together until combined. Set aside.

Sift the cake flour, sugar, baking powder, and salt in the bowl of a stand mixer. (Or if using a handheld mixer, any large mixing bowl.) With the paddle attachment, beat the ingredients together on low speed for a few seconds to gently combine. Add the butter, vanilla, and about 1/2 of the lavender milk/sour cream mixture. Mix on medium speed until the dry ingredients are moistened, about 1 minute. Stop the mixer and scrape down the sides and up the bottom of the bowl.

Whisk the eggs into the remaining lavender milk/sour cream mixture. With the mixer running on medium speed, add the egg mixture in 3 additions, mixing for about 15 seconds after each addition. Stop the mixer and scrape down the sides and up the bottom of the bowl, then mix for about 15 more seconds until batter is completely combined. Avoid over-mixing. Some small lumps are OK.

Pour batter evenly into cake pans. If desired, weigh them to ensure accuracy. Bake for around 20-22 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before leveling and frosting.

Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Generously brush lavender syrup on the top of each cake. I use every last drop!

In a large bowl using a hand-held mixer or stand mixer fitted with a whisk or paddle attachment, beat the butter and cream cheese together on medium speed until creamy and smooth, about 2 minutes. Add confectioners’ sugar, milk, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 minutes. Add more confectioners’ sugar if frosting is too thin, more milk if frosting is too thick, or an extra pinch of salt if frosting is too sweet. Frosting should be spreadable and creamy, not runny. If desired, divide frosting in half and tint half light purple. I used 2 drops of this mauve food coloring.

Place 1 cake layer on your cake stand, cake turntable, or serving plate. Spread a thin layer of white frosting on top. Fill a piping bag fitted with a large round tip (I used Wilton 1A piping tip) with 1/2 cup of white frosting. (Or use a zipped-top bag with the corner snipped off.) Pipe a ring around the outer top edge of the cake to create a “dam.” Pipe a second ring of white frosting a couple inches in from the outer ring to create a “bull’s eye.” Spoon and spread blackberry jam in the gaps. Top with 2nd cake layer and repeat, then place 3rd cake layer on top. If there’s any remaining, spread a thin layer of white frosting all over the top and sides of cake to create a crumb coat. Chill in the refrigerator for 15 minutes. Finally, spread the purple frosting all over the top and sides. I use and recommend an icing spatula to apply the frosting. If there’s any extra frosting, pipe decor on top. I piped a swirled border with Wilton 1M piping tip. Garnish with blackberries.

Refrigerate cake for at least 20 minutes before slicing and serving. This helps the cake hold its shape when cutting.

Cover leftover cake tightly and store in the refrigerator for up to 5 days.