I recommend making the jam the night before you want to make the macarons so that it has time to fully cool.
Add all of the jam ingredients to a large saucepan over medium-low heat. Allow the jam to simmer for 20-30 minutes, stirring frequently until thickened.
Transfer the jam to a glass jar and allow to cool to room temperature. Store in the fridge for up to two weeks.
Sift together almond flour and powdered sugar into a large bowl and set aside.
In a stand mixer fitted with the whisk attachment, whisk the egg whites on medium speed until they become foamy, then add in the cream of tartar.
Turn the mixer up to medium-high and gradually add in the granulated sugar (make sure to add the sugar in very slowly so that the meringue doesn’t deflate). Once all the sugar is added, turn the mixer to the highest setting and whisk on high until soft peaks form. Add in 2-3 drops of lavender extract paste and then continue to whisk on high until stiff peaks form.
Using a spatula, gently fold in half of the dry ingredient mixture until fully combined. Then gently fold in the remaining dry ingredients. Continue to gently fold the batter until it reaches a “flowing lava” consistency. You should be able to make a “figure 8” with the batter before it breaks. As soon as you reach this consistency, stop mixing! If you over-mix the batter, the macarons won’t rise properly in the oven.
Line a large baking sheet with a silicone mat or parchment paper. Scoop batter into a large piping bag fitted with a round tip. Hold the piping bag perpendicular to the silicone mat and pipe 1 inch rounds. Pick up the baking sheet and drop it onto the counter 3-4 times to help bring any air bubbles to the surface of the macarons. Then use a toothpick to pop any visible air bubbles to make a smooth surface.
Let the macarons sit at room temperature for 30-40 minutes or until a skin forms on the surface. You should be able to lightly touch the surface without any batter being disturbed.
While the macarons are resting, preheat the oven to 300°F.
Bake one sheet at a time on the center rack of the oven for 15 minutes or when they do not move on their “feet” when touched.
Let the macarons cool completely on the baking sheet before removing them. They should easily peel off when fully cooled. Match up your shells before filling, and then turn one side over.
Blackberry Buttercream Filling
In a large bowl using a handheld or stand mixer fitted with a whisk attachment, cream the butter and powdered sugar together until smooth.
Add in about 2 teaspoons of your cooled blackberry jam, then beat on high speed for 3-4 minutes or until the buttercream is super light and fluffy.
Once the macarons are the blackberry jam is cooled, pipe the buttercream around the edge of the macaron shell. Then add a small scoop of blackberry jam in the center (about ½ teaspoon). Then top with the other shell.
Put the assembled macarons in an airtight container, then place them in the fridge for mature for 12-24 hours before eating for the best taste/texture. Or just eat them all immediately. You do you.