While the cake is cooling, prepare the jam: In a medium saucepan, stir together the mashed raspberries, sugar, lemon zest, and lemon juice. Place the pan on the stove at a medium-high heat and bring to a full boil for about a minute, stirring constantly. Reduce the heat to medium/low and allow to bubble and thicken, stirring occasionally. As the jam thickens, the bubbles will get smaller. Stir in 1 Tbsp of chia seeds. If using a candy thermometer, allow the mixture to bubble until it reaches 220°F. If not using a thermometer, coat the back of a spoon with the jam–if you can run finger through it and a clear path is left, the jam is ready to remove from the heat.