Clear a level space in a freezer kept at 0°F (−18°C) or below. Line a large rimmed baking sheet with parchment. Rinse berries under running water, drain and dry thoroughly with a clean towel. Hull and slice strawberries thinly; halve very large blueberries.

Stir Greek yogurt, maple syrup and vanilla until smooth. Spread on the parchment into a rectangle about 10 × 16 inches (25 × 40 cm), about 1/4 inch (6 mm) thick. Keep the thickness even rather than spreading very thin edges around a thick center. If the tray will not fit, divide the mixture between smaller lined trays while keeping the same layer thickness.

Scatter strawberry slices, blueberries and chocolate chips evenly over the yogurt, leaving a little yogurt visible between toppings. Press them gently into the surface. Cover loosely without touching the yogurt and freeze at least 4 hours, until fully firm and brittle. Freezer temperature, layer thickness and fruit affect the time.

Lift the frozen sheet by the parchment and break into about 12 similar-sized pieces. For more comparable portions, divide the whole frozen batch by weight into 12. Serve a few pieces straight from the freezer; let an individual piece stand briefly if too hard to bite. It softens quickly.

Immediately pack the remaining pieces in an airtight freezer-safe container with parchment between layers. Keep at 0°F (−18°C) and plan to use within 2 weeks for a fresher texture. Take out only what you will eat. Do not save perishable portions left out more than 2 hours, or 1 hour above 90°F (32°C).