Take a cutting board and knife. Rinse 4 small plums and dry. Roughly chop those plums (about 230 grams) and set aside
Take 1/2 of a lemon and zest with a microplane and juice with a handheld juicer. You can skip the step on lemons if you prefer to use lemon extract. Set aside
Preheat the oven to 350℉/177℃ and gather the ingredients. Line a 12-cupcake mold pan with cupcake liners and set aside
In a standing mixer with a paddle attachment or a large bowl with a mixer, beat olive oil and sugars together
Add one egg and beat until just incorporated. Follow with the second egg, vanilla extract, lemon juice or lemon extract. Repeat the process.
In a separate bowl, whisk together the flour, rosemary, lemon zest, salt, and baking powder
Add the dry ingredients to the wet ingredients in the standing mixer and mix on medium until just incorporated
Add milk and mix again until just incorporated
With a rubber spatula, fold in the plums until they are mostly distributed throughout batter
Spoon batter into cupcake liners
Rinse and dry standing mixer bowl or use the large bowl that the dry ingredients were in if you are using a handheld mixer. We are about to make the streusel, which will go on top of the muffin batter
Combine butter, flour, sugar and cinnamon in standing mixer bowl with a paddle attachment or in a large bowl with a hand-held mixer on low speed. We just want to mix enough to get a crumbly, coarse texture.
Spoon the streusel on top of each muffin batter
Bake in the oven for 50 minutes. If the toothpick doesn't come out clean when you insert it to test for doneness, bake for another two minutes at a time
Allow the muffins to cool before serving. Enjoy!