Preheat the oven to 350°F. Line a 9x13-inch baking dish with parchment paper, leaving an overhang for easy removal.
In a stand mixer with the whisk attachment (or a hand mixer), beat cold butter and sugar on medium speed until light and creamy. Scrape the sides as needed. Add vanilla extract, zest of 1 lemon (if using), and salt, then mix to combine. Add flour and mix until just combined into a thick dough. Do not overmix.
Press a little over half the dough into the prepared pan. Spread raspberry jam evenly over the dough, leaving a 1/4-inch border around the edges.
Mix the remaining dough with granola until combined. Drop small pieces of the mixture over the jam. Sprinkle sliced almonds evenly on top.
Bake for 42–45 minutes, or until the top is golden brown and the almonds are toasted. Remove from the oven and let cool completely in the pan on a wire rack. (The bars need to fully set before cutting.)
Use the parchment paper overhang to lift the bars from the pan. Place on a cutting board and slice into squares. Enjoy soon after making (the jam softens the shortbread over time).
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Notes
Note 1: The better the butter, the better the flavor! Cold butter creates the best texture—stand mixers handle it easily, but hand mixers may require extra effort.Note 2: Good-quality jam makes a difference! Use any flavor you like. Go seedless for a smoother filling.Note 3: Any kind without dried fruit or chocolate works well.Note 4: Regular sliced almonds are great. I’ve also made these bars with honey almonds, and that was so good!Storage: Keep cooled bars in an airtight container at room temperature for 1-3 days. They’re best early on before the jam softens the bars. To freeze, wrap bars individually and store for up to 3 months (texture may change).