Zest and juice both lemons – you should have 1/4 cup of juice. Place the zest, juice and raspberries in a small pot and heat on medium low, using the back of a spoon to crush the berries, till the mixture is liquid.

Pour the mixture into a fine sieve set over a large bowl, pressing hard to extract all of the liquid (be sure to scrape the liquid from the bottom of the sieve into the bowl as well). Discard the solids and return the liquid in the bowl to your pot. Place the butter into the bowl, clean the sieve and place it over the bowl.

Add the sugar to the pot and whisk to combine. While continually whisking add the eggs, one at a time, along with the cornstarch. Cook the mixture over medium heat, stirring constantly, until it thickens (it will coat the back of a wooden spoon). Immediately remove from the heat.

Pour the mixture through the sieve and press down to extract all of the liquid. Whisk until the butter is melted and completely incorporated into the liquid. Transfer to a jar and refrigerate till needed.

Pre-heat the oven to 350 degrees and line an 8" square baking pan with parchment paper, leaving an overhang on all sides.

Beat the butter and sugar till light and fluffy. Add the eggs, vanilla and sour cream and beat again. Add the flour, baking powder, baking soda, salt, lemon zest and spicy baking blend, and beat till combined.

Transfer the batter to your prepared pan and use a small angled spatula to spread evenly. Top with the curd and spread evenly. Top with the streusel.

Bake for 50 - 60 minutes until a tester tests clean (no crumbs) and the center feels firm when lightly pressed (if the edges are starting to get too brown you can cover with strips of foil). Remove from the oven and cool in the pan on a wire rack.