Preheat the oven to 400 degrees F and place a rack in the top 1/3 of the oven, at least 6 inches from the heat source. Grease a regular muffin tin or use muffin liners.
To make the streusel, in a small bowl whisk to combine the flour, brown sugar and cinnamon. Use your fingers to work the 2 tablespoons of butter into the mixture until it resembles wet sand. Use your fingers to clump it together. Cover and set in the fridge to chill until ready to use.
In an electric stand mixer fitted with the paddle attachment add the butter, granulated sugar and brown sugar and mix on medium until light and fluffy, about 4-5 minutes. With the mixer on low, add in the eggs, one at a time, making sure each is well blended before adding in the next. Scrape down the sides and bottom of the bowl to make sure everything is incorporated. Add in the lemon zest and vanilla and run the mixer for 1 minute more.
In a medium bowl whisk together the flour, baking powder, baking soda, cinnamon, sea salt and nutmeg. With the mixer on low, alternate adding the flour mixture and the milk in two additions, ending with the milk. When it is just blended, take the bowl out of the mixer and scrape the sides and bottom of the bowl with a spatula to make sure everything is combined. ***At this point, you can wrap the bowl tightly and stash in the fridge for several hours or overnight...because fresh raspberry jam streusel muffins in the morning (with little effort) is called living your best life.
Scoop 1 ½ tablespoons of batter into the muffin liners and top with 1 teaspoon of raspberry jam in the center. The batter is thick, so press the jam in to push the batter to cover the bottom of the muffin cup. Top with another 1 ½ tablespoons of batter and ½ teaspoon-ish additional raspberry jam. The muffin cups will be just over ¾ full. Let the muffins sit for at least 15 minutes before putting in the oven; this helps them get a wee more rise (and is optional).
Sprinkle the tops with the streusel (pinching it into clumps before dropping on the muffin) and bake for 18-22 minutes in the top third of the oven or until the tops spring back when gently pressed. You want them to be puffed up and slightly golden. For best results, bake one tin at a time and rotate midway through baking. Let cool in the muffin tin for about 10ish minutes, then use a butter knife to help get them out to finish cooling on a rack. Store airtight at room temperature…they’ll keep for a couple of days but I liked them best the same day they were baked.