In a heat-proof bowl, beat the egg yolks for 1 minute. Add the sugar and beat 1 minute more. Stir in the lemon juice and zest.

Create a double boiler by placing the heat-proof bowl over a pot of simmering water, being sure the bottom of the bowl isn't touching the water. Adjust heat as needed to maintain a gentle simmer. Cook the egg mixture over the simmering water, stirring nearly constantly, until it reduces and thickens, about 10–20 minutes. (The curd has thickened enough when the spoon leaves a trail.)

Stir in the butter 1 tbsp at a time, allowing to melt completely before adding more. Remove the curd from the heat and pour through a fine-mesh sieve into a storage jar, using a spoon to work the curd through.

Cool to room temperature, then cover and refrigerate until set, at least 2 hours.

In a small pot over medium-low heat, simmer the cream, milk, sugar, salt, and lavender until the sugar dissolves, about 5 minutes. Reduce heat to low and let stand 10 minutes more to infuse the lavender. Remove from the heat.

In a medium bowl, whisk the yolks for 1 minute. Slowly whisk 1/3 of the hot cream mixture into the yolks, then whisk the yolk mixture back into the pot with the cream mixture.

Return the pot to medium-low heat, and cook while whisking to 170F. The mixture will slightly thicken, and should coat back of a spoon.

Strain the mixture through a fine mesh sieve into a clean bowl and allow to cool to room temperature. You may add food coloring at this stage-- be sure to stir REALLY well if you use gel food coloring. Cover and chill in the refrigerator at least 4 hours, or up to overnight.

Churn the lavender ice cream base according to machine instructions. Immediately after churning, fold in the lemon curd, leaving ribbons of curd throughout. Spread into a large pan or pack into freezer-safe containers and freeze to set.